Add Func::hash_key and Table::hash_key methods (bytecodealliance#7086)

* Wasmtime: Move `Global` and `Table` to `externals` submodules

Just mechanical code motion, not functional changes.

* Wasmtime: Add `hash_key` methods for `Func` and `Table`

* Rename `wasmtime::Func::caller_checked_func_ref` to `wasmtime::Func::vm_func_ref`

We removed the "caller checked" part of the `VMFuncRef` type's name a while ago,
so update this method to be in line with that.

No functional changes, just mechanical renaming.

* cargo fmt

* Fix doc link

Author: fitzgen