In preparation for eventual support for wasm interface types this commit
moves around a few panics internally inside of conversions between the
`wasmtime` crate and the underlying jit support crates. This should have
any immediately-visible user changes, but the goal is that this'll help
support interface types which means `wasmtime` will have types that are
not supported by wasmtime itself and we'll be able to more gracefully
support that with error messages instead of accidental panics.
a45b037bfc