This commit introduces two new methods on `InstanceAllocator`:
* `validate_module` - this method is used to validate a module after
translation but before compilation. It will be used for the upcoming pooling
allocator to ensure a module being compiled adheres to the limits of the
allocator.
* `adjust_tunables` - this method is used to adjust the `Tunables` given the
JIT compiler. The pooling allocator will use this to force all memories to
be static during compilation.
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.