This commit fixes a panic related to type imports where an import of a
type didn't correctly declare the new type index on the Wasmtime side of
things. Additionally this plumbs more support throughout Wasmtime to
support type imports, namely that they do not need to be supplied
through a `Linker`. This additionally implements a feature where empty
instances, even transitively, do not need to be supplied by a Wasmtime
embedder. This means that instances which only have types, for example,
do not need to be supplied into a `Linker` since no runtime information
for them is required anyway.
Closes#5775
b5e9fb710b