* Remove seemingly unneeded const on MetadataKey<T> new
* Change biome temperature/moisture to integer, x100 to remove floating-point so can use within stable 'const fn'
* Remove unstable const_fn feature, now using stable const fn: see https://www.reddit.com/r/rust/comments/9msqfn/const_fn_soon_on_stable_rust/
* Test on Rust beta (awaiting 1.31 release for stable)
* Update readme for beta Rust support