👻 Add options to customize behavior of unknown Wasm imports

[acfoltzer]

Adds a new shared option to the CLI with three behavior modes:

      --unknown-import-behavior <UNKNOWN_IMPORT_BEHAVIOR>
          Set the behavior for unknown imports.

          Note that if a program only works with a non-default setting for this flag, it is unlikely to be publishable to Fastly.

          [default: link-error]

          Possible values:
          - link-error:   Unknown imports are rejected at link time (default behavior)
          - trap:         Unknown imports trap when called
          - zero-or-null: Unknown imports return zero or a null pointer, depending on the type

This ended up touching more files than I'd expect, because I had to make the integration test harness bubble out link-time errors rather than .expect()ing them, so there are quite a few new ?s across the integration test suite.

[tedmielczarek-fastly]

This sounds great! Since we're using a privileged hostcall in our internal service, we currently have a cargo viceroy feature to avoid referencing that hostcall for local testing. I think the zero-or-null behavior would work perfectly there and make local testing less annoying.

[iximeow]

.... it occurs to me that this is a fascinating way to discover at runtime if a program's dependencies really do need browser-shaped wasm glue, or if maybe you'd never actually reach any of that at runtime. hmmmm..

very handy!

[awortman-fastly]

i should approve from the right account though