0.2.1

This release is mostly small tweaks, but enough performance improvements have piled up that it seems worthwhile!

• Changed fidget::eval::Vars to borrow instead of use an Arc
• Properly pre-allocated mmap regions based on estimated size
• Bump dependencies, fixing a Dependabot warning about atty being unmaintained
• Fixed performance regressions in 3D rendering
• Added wasm32-unknown-unknown target to CI checks
• Reordered load/store generation so that register allocation generates linear code ("linear" in the type-system sense).
• Small optimizations to register allocation
• Added unit tests for rendering correctness