Exported imports

[awortman-fastly]

https://github.com/fastly/lucet/commit/1f5839b254b32e46dc67800f73994a6e42d2b093 adds a test in entrypoint to reach an exported import by its export name, which is something we ought to support, but don't. oops.

Related, if (import "env" "add_4_hostcall" (func $add_4_hostcall (param i64 i64 i64 i64))) is also added before the export it produces a duplicate import of add_4_hostcall, and throws off all tests on that module by having function pointers off by one with respect to function indices.

These two can probably grow into a wider set of runtime tests around import/export support as that grows (for globals, memory, ..)