Add an assertion that a HostFunc's store agrees on engines

This commit adds an assertion which was previously forgotten when
inserting a `HostFunc` into a `Store`. This can happen when a `Linker`
is defined with one engine but it's used to interoperate with a store
defined within a different engine.

A function contains type information that's only valid relative to the
engine that it was defined within. This means that if a function is used
within a different engine then type information may look valid when in
fact it is not. For example it's otherwise possible to insert a function
into an engine with one type and call it in a different engine with a
different type.

Similar to how `Store` misuse is a panic throughout `wasmtime`'s API
this commit also turns this behavior into panic, so there's no API
impact. Documentation has been updated accordingly to indicate that
various functions on `Linker` will panic if a `store` is provided that's
connected to a different `Engine`.

tests/all/linker.rs

@@ -256,7 +256,7 @@ fn get_host_function() -> Result<()> {
 
     let mut linker = Linker::new(&engine);
     linker.func_wrap("mod", "f1", || {})?;
-    let mut store = Store::<()>::default();
+    let mut store = Store::new(&engine, ());
     assert!(linker
         .get_by_import(&mut store, &module.imports().nth(0).unwrap())
         .is_some());