geohash-rs-wasm

Wraps the geohash-rs crate, with a few judicious additions:

• Vectorized versions of the string form decode and encode with one-copy passing
• Integer form decode and encode methods (also vectorized)
• Vectorized round-trip string->int encoding/decoding (i.e. 'wxy321' -> 3465345221n)

NB: there is a small amount of unsafe code in the vectorized methods, involving pointer dereferencing.

Roadmap:

• [x] decode_int, encode_int for u32, u64 variants (vectorized)
• [] decode, encode for u32, u64 variants (vectorized)
• [] simd optimizations
• [] wasm feature fallbacks/multiple targets
• [] passthrough upstream non-vectorized methods
• [] expand environment targets