This exposes the functionality of the `Linker` type where a
store-independent function can be created and inserted, allowing a
linker's functions to be used across many stores (instead of requiring
one linker-per-store).
Closes#3110
Implement Wasmtime's new API as designed by RFC 11. This is quite a large commit which has had lots of discussion externally, so for more information it's best to read the RFC thread and the PR thread.