[OutputByteStream] Write to buffer directly from only few methods
[OutputByteStream] Write to buffer directly from only few methods

This will help implementing the algorithm to empty the buffer when its full. With this change only `write(byte: UInt8)`, `write(bytes: [UInt8])` and `write(ptr: UnsafeBufferPointer<UInt8>)` directly updates the buffer.
apple/swift-package-manager,mxcl/swift-package-manager,mxcl/swift-package-manager,abertelrud/swift-package-manager,vmanot/swift-package-manager,abertelrud/swift-package-manager,JGiola/swift-package-manager,JGiola/swift-package-manager,JGiola/swift-package-manager,vmanot/swift-package-manager,JGiola/swift-package-manager,apple/swift-package-manager,apple/swift-package-manager,sschiau/swift-package-manager,haskellswift/swift-package-manager,sschiau/swift-package-manager,haskellswift/swift-package-manager,vmanot/swift-package-manager,mxcl/swift-package-manager,JGiola/swift-package-manager,sschiau/swift-package-manager,apple/swift-package-manager,sschiau/swift-package-manager,abertelrud/swift-package-manager,abertelrud/swift-package-manager,vmanot/swift-package-manager,apple/swift-package-manager,haskellswift/swift-package-manager,haskellswift/swift-package-manager,sschiau/swift-package-manager,vmanot/swift-package-manager,abertelrud/swift-package-manager,mxcl/swift-package-manager