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
Currently I don't know how we can reasonably implement this. Given all
the signatures of how we call functions and how functions are called on
the host there's no real feasible way that I know of to hook these two
up "seamlessly". This means that a component which reexports an imported
function can't be run in Wasmtime.
One of the main reasons for this is that when calling a component
function Wasmtime wants to lower arguments first and then have them
lifted when the host is called. With a reexport though there's not
actually anything to lower into so we'd sort of need something similar
to a table on the side or maybe a linear memory and that seems like it'd
get quite complicated quite quickly for not really all that much
benefit. As-such for now this simply returns a first-class error (rather
than the current panic) in situations like this.
