Add ability to map dirs by name

[wingo]

The wasi-testsuite used to run tests from a current working directory such that the dirs to expose to the guest have the same name to the guest and host, i.e. that --dir foo exposes /foo to the guest and expects that ./foo exists. However I am refactoring to pass mappings explicitly: i.e. to be able to expose /tmp/foo on the host as bar on the guest. This allows me to print a list of commands that failed at the end of a test run.

Wasmtime already supports this, via the odd syntax --dir /path/to/host/thing::guest-location/possibly-with-subdirs/etc. WAMR supports it too, calling the option --map-dir. Would it be possible to add support for this to Wizard? :)

[titzer]

Yeah, it makes sense to allow this.