This commit moves all of the caching support that currently lives in `wasmtime-environ` into a `wasmtime-cache` crate and makes it optional. The goal here is to slim down the `wasmtime-environ` crate and clearly separate boundaries where caching is a standalone and optional feature, not intertwined with other crates.
This is the wasmtime-environ crate, which contains the implementations
of the ModuleEnvironment and FuncEnvironment traits from
cranelift-wasm. They effectively
implement an ABI for basic wasm compilation that defines how linear memories
are allocated, how indirect calls work, and other details. They can be used
for JITing, native object files, or other purposes.