Assorted minor data updates

[jmschonfeld]

This PR contains a handful of small, assorted updates to Data's implementation to clean up some of the implementation and prepare it for further evolution. The changes include:

• Using withUnsafeTemporaryAllocation instead of a custom inline buffer tuple
• Updating import statements to fix build errors when building for Darwin
• Updating deprecated APIs to use other equivalent public API entry points instead of internal _Representation entrypoints
• Move copyBytes(to:) (a requirement of the DataProtocol protocol) to the DataProtocol file instead of the main Data.swift
• Avoid calling .count twice on a buffer
• Update Data's custom mirror representation to match the children size of Array (up to 256 elements)