元のソースをすこしアップグレードして、swift3b6への適用も完了しました。
https://gist.github.com/singcodes/15614c428b83d8eb47dd42f3de1bf4cd

###swift 3

mutable関数を削除し、全ての機能をcast関数で含めました。
pointeeの短いバージョンのo変数を追加しました。
- mutable pointerの場合setが出来、ただのpointerの場合、エラーなしで無視されます。
- ただし、無視されるのはswift的のことではないので、どう修正すればいいか考えています。できれば、protocolのextension中で解決したいんですが…いい考えがある方は是非お願いします。
raw pointerへの変更も出来るように、castRaw関数を追加しました。

//
//  Castable.swift
//  Castable
//
//  Created by jayios on 2016. 6. 23..
//  Copyright © 2016년 ngenii. All rights reserved.
//

import Foundation
import CoreGraphics

public protocol PointerCastable {}

public protocol UnsafePointerable {
    init(_ other: UnsafePointer<Pointee>)
    init?(_ other: UnsafePointer<Pointee>?)
    init(_ other: UnsafeMutablePointer<Pointee>)
    init?(_ other: UnsafeMutablePointer<Pointee>?)
    associatedtype Pointee
    var pointee: Pointee { get }
    func withMemoryRebound<T, Result>(to: T.Type, capacity count: Int, _ body: (UnsafeMutablePointer<T>) throws -> Result) rethrows -> Result
}

public protocol PointerShortener {
    associatedtype Pointee
    /// meaning for o(bject)
    var o: Pointee { get set }
    var pointee: Pointee { get }
}

extension PointerShortener where Self: UnsafePointerable {
    public var o: Pointee {
        set {
            if let mutable = self as? UnsafeMutablePointer<Pointee> {
                mutable.pointee = newValue
            }
        }
        get {
            return self.pointee
        }
    }
}

extension UnsafePointer: PointerCastable, UnsafePointerable, PointerShortener {}
extension UnsafeMutablePointer: PointerCastable, UnsafePointerable, PointerShortener {}

public protocol UnsafeRawPointerable{
    init<T>(_ other: UnsafePointer<T>)
    init?<T>(_ other: UnsafePointer<T>?)
    init<T>(_ other: UnsafeMutablePointer<T>)
    init?<T>(_ other: UnsafeMutablePointer<T>?)
}

extension UnsafeRawPointer: UnsafeRawPointerable, PointerCastable {}
extension UnsafeMutableRawPointer: UnsafeRawPointerable, PointerCastable {}

extension PointerCastable where Self: UnsafePointerable {
    public func cast<P: UnsafePointerable, M>() -> P? where P.Pointee == M {
        if self is P {
            return self as? P
        }
        let ptr = self.withMemoryRebound(to: M.self, capacity: MemoryLayout<M>.stride){$0}
        return P(ptr)
    }
    public func cast<P: UnsafePointerable, M>() -> P where P.Pointee == M {
        if self is P {
            return self as! P
        }
        let ptr = self.withMemoryRebound(to: M.self, capacity: MemoryLayout<M>.stride){$0}
        return P(ptr)
    }
    public func castRaw<R: UnsafeRawPointerable, T>(from: T.Type) -> R? {
        if R.self == UnsafeRawPointer.self {
            let ptr: UnsafePointer<T> = self.cast()
            return R(ptr)
        }
        let ptr: UnsafeMutablePointer<T> = self.cast()
        return R(ptr)
    }
    public func castRaw<R: UnsafeRawPointerable, T>(from: T.Type) -> R {
        if R.self == UnsafeRawPointer.self {
            let ptr: UnsafePointer<T> = self.cast()
            return R(ptr)
        }
        let ptr: UnsafeMutablePointer<T> = self.cast()
        return R(ptr)
    }
}

extension PointerCastable where Self: UnsafeRawPointerable {
    public func cast<P: UnsafePointerable, M>(to: M.Type) -> P where P.Pointee == M {
        if self is UnsafeRawPointer {
            let raw = self as! UnsafeRawPointer
            let ptr = raw.assumingMemoryBound(to: M.self)
            return P(ptr)
        } else {
            let raw: UnsafeMutableRawPointer = self as! UnsafeMutableRawPointer
            let ptr = raw.assumingMemoryBound(to: M.self)
            return P(ptr)
        }
    }
}

###swift 2
Usage:

let a = malloc(sizeof(Int)) -> Intのサイズを持つUnsafePointer<Void>を作成
let b: UnsafeMutablePointer<Int> = a.mutable() -> UnsafeMutablePointer<Int>にキャストされます。

gist url

protocol Castable {}

protocol UnsafePointerProtocol: NilLiteralConvertible {
    associatedtype Memory
    init(nilLiteral: ())
    init<Memory>(_ from: UnsafeMutablePointer<Memory>)
    init<Memory>(_ from: UnsafePointer<Memory>)
    var memory: Memory { get }
    func mutable<M>() -> UnsafeMutablePointer<M>
}

extension UnsafeMutablePointer : UnsafePointerProtocol, Castable{}
extension UnsafePointer : UnsafePointerProtocol, Castable{}

extension UnsafePointerProtocol where Self: Castable {
    func cast<P: UnsafePointerProtocol, M where M == P.Memory>() -> P {
        switch self {
        case let ptr as UnsafePointer<Memory>:
            return P(ptr)
        case let ptr as UnsafeMutablePointer<Memory>:
            return P(ptr)
        default:
            return nil
        }
    }
    
    func mutable<M>() -> UnsafeMutablePointer<M> {
        switch self {
        case let ptr as UnsafePointer<Memory>:
            return UnsafeMutablePointer<M>(ptr)
        case let ptr as UnsafeMutablePointer<Memory>:
            return UnsafeMutablePointer<M>(ptr)
        default:
            return nil
        }
    }
}


protocol Pointerable {
    associatedtype Element
    var pointer: UnsafePointer<Element> {get}
}

extension Array : Pointerable {}
extension ArraySlice : Pointerable {
    func array() -> Array<Element> {
        return Array<Element>(self)
    }
}

extension Pointerable where Self: SequenceType {
    var pointer: UnsafePointer<Element> {
        switch self {
        case let a as Array<Element>:
            return UnsafePointer<Element>(a)
        case let s as ArraySlice<Element>:
            return UnsafePointer<Element>(s.array())
        default:
            return nil
        }
    }
}