This commit syncs tests with latest wasmtime revision.
As such, it now utilises the `wasmtime-api` crate for
runtime setup.
Closes#126, #127, #128, #129.
Changes:
* use [tempfile] crate for auto mgmt of temp dirs
* use concrete types in place of generics in `utils` module
[tempfile]: https://github.com/Stebalien/tempfile