cloned from:

• SR-5363 Update Foundation.Data API for UnsafeRawBufferPointer.

relates to:

• SR-9719 Foundation Data API: add copyBytes methods taking UnsafeMutableRawPointer

Issue Description:

In the Foundation Data API, add methods that copy into a raw buffer pointer:

public func copyBytes(to buffer: UnsafeMutableRawBufferPointer, from range: Range<Index>? = nil) -> Int
public func copyBytes(to buffer: UnsafeMutableRawBufferPointer, count: Int)

The raw buffer form is more general than this typed API:

public func copyBytes<DestinationType>(to buffer: UnsafeMutableBufferPointer<DestinationType>, from range: Range<Index>? = nil) -> Int

However, we don't yet have language support for automatic typed-to-raw buffer argument conversion, so the typed buffer API is still a useful convenience.

The behavior of the API that takes a `count` instead of a `range` should behave the same as this existing API, except that it will be bounds checked in Debug builds:

public func copyBytes(to buffer: UnsafeMutablePointer<UInt8>, count: Int)

These overloads should be completely source compatible, but I have not verified with a prototype.