Big things:
- Support for targets without hardware SIMD
- Support for architectures other than x86
- Documentation & examples for most objects
- Intuitive support for uneven collections
- Support SIMD-accelerated reductive operations
- Add upcasting, casting, and downcasting

Smaller things:
- Add summation and product calculations for all vectors
- Add default initializer for vectors
- Add many more polyfills and feature gates
- Allow scalar iteration of SIMD iterators with `map` and `fold`
- Add vector constructors for interleaved and half-and-half patterns
- Expose saturating addition and subtraction intrinsics

Maintainence & Bugfixes:
- Add tests for sqrt, transmute, abs, recip, upcast, downcast, cast, and more
- Make [saturating]_{hadd, hsub} portable