chore: update to latest wasmtime release (0.38.0) (#261)

This updates to `wasmtime` v0.38.0.  Because module linking support has been
removed from `wasmtime` as of v0.36.0, and the component model implementation is
not yet mature enough to use as a replacement (see
bytecodealliance/wasmtime#4303 for details), it is
necessary to temporarily disable the Spidermonkey tests.  I've done this by
commenting-out the broken code in `instantiate_smw`, prefixed its name with and
underscore, and renamed the relevant host.rs files to ignore_host.rs to hide
them from the test infrastructure.

Once the component model implementation is mature enough and the broader
ecosystem has been updated to use it, we can update and restore the Spidermonkey
tests.

fixes bytecodealliance/wit-bindgen#259

Signed-off-by: Joel Dice <[email protected]>