WASM: Support for setting an imported function's module name

[pkedy]

I need to compile a WebAssembly program that imports functions from a host that uses a module namespace other than the standard env. I enhanced go:export to support this. For example, in addition to the default export (env module)

//go:export multiply
func multiply(x, y int) int {
    return x * y;
}

You can set the module using //go:export [module] <name>

//go:export math multiply
func multiply(x, y int) int {
    return x * y;
}

Alternatively, you can use a separate comment //go:module <name> in conjunction with go:export or go:linkname. I'm happy to make any tweaks to the argument format.

I tried to run make test but got clang linker errors. I tested using go install and wasm2wat to print the imports of the compiled wasm.

I believe this fixes #423.

[CryZe]

It doesn‘t fix the issue, but it‘s a large step towards it.

[pkedy]

Oops. I need to reopen this targeting the dev branch.

[pkedy]

Reopened in #455