wasm: implement the //go:wasmimport directive

[aykevl]

It is implemented upstream almost implemented upstream and looks pretty stable.

@neelance I'd like to get your opinion on this.
This would mean that TinyGo implements the //go:wasmimport directive for all packages (not just runtime and syscall/js). This seems mostly safe to me, and certainly better than the current //go:wasm-module directive that we're using. What do you think, am I missing something?
The ABI will be mostly the same as the one used in the Go toolchain, with minor differences here and there. It roughly matches the C ABI.

[codefromthecrypt]

fyi, this isn't quite in upstream, yet, though I personally support using this anyway https://go-review.googlesource.com/c/go/+/350737

[aykevl]

fyi, this isn't quite in upstream, yet

My bad, thanks for correcting.

[gedw99]

basic question...

//go:wasmimport directive is for Browser side or server side ? Or both ?

I wonder if this will also need to catchup ? chrome://extensions/?id=pdcpmagijalfljmkmjngeonclgbbannb
It is the chrome dwarf debugger extension used to debug tinygo in the Chrome Browser.

[anuraaga]

golang/go@02411bc has landed so hopefully this can move forward :)

[deadprogram]

Reminder to @aykevl about this one 🛩️

[aykevl]

Updated and removed the commits from #3142 from this PR.

[deadprogram]

We also need to update examples/wasm/export and the associated README with this change. I can submit a separate PR.

Thanks everyone for working on this, exciting to see this moving forward! Now merging.