Strip Foundation imports

[STREGA]

Currently every supported platform supports Foundation, but Bare Metal is coming and probably won't. Confine Foundation to Platform instances and replace it's functionality with abstractions.

• [x] Add @_exported import for Data and URL to GateEngine.swift
  • we'll use SwiftFoundation for these when it's ready
• ~GameMath uses sin functions. Look into implementing native ones, possibly in C/C++~ _{C and C++ standard libraries both have sin functions, so Foundation should always have them.}