Make `gcore` a std library, remove `std`, `alloc` and more unused features

[Firestar99]

Requires https://github.com/GraphiteEditor/Graphite/pull/2729

This PR contains the code changes, for bulk renames see https://github.com/GraphiteEditor/Graphite/pull/2735

Features removed:

• std, alloc to make it std-only
  • std depends on reflections, spectra, rustybuzz, image and ctor
  • alloc depends on dyn-any and bezier_rs
• gpu was for rust-gpu compiling
• log, rand_chacha and kurbo are required for it to even compile