Add a VMComponentContext type and create it on instantiation (#4215)

* Add a `VMComponentContext` type and create it on instantiation

This commit fills out the `wasmtime-runtime` crate's support for
`VMComponentContext` and creates it as part of the instantiation
process. This moves a few maps that were temporarily allocated in an
`InstanceData` into the `VMComponentContext` and additionally reads the
canonical options data from there instead.

This type still won't be used in its "full glory" until the lowering of
host functions is completely implemented, however, which will be coming
in a future commit.

* Remove `DerefMut` implementation

* Rebase conflicts

crates/runtime/src/lib.rs

@@ -30,6 +30,8 @@ use wasmtime_environ::DefinedMemoryIndex;
 use wasmtime_environ::FunctionInfo;
 use wasmtime_environ::SignatureIndex;
 
+#[cfg(feature = "component-model")]
+pub mod component;
 mod export;
 mod externref;
 mod imports;