`wasmtime::CoreDump::_serialize` shouldn't need a generic `T` in its store

[fitzgen]

Follow up to #7078 (comment)

It is there because the method calls public APIs that require a store with a T but none of them actually use the T. For every pub fn foo<T> that is called, we could create a pub(crate) fn _foo without the T and then call those methods from _serialize and ultimately remove the T from _serialize.

This would make it so that calling wasmtime::CoreDump::serialize isn't monomorphized for every T in a store passed to it, which can help compile times when there is more than one T used in the program.