Use unsafe.Pointer instead of types not supported by Go in wasm{im,ex}port

Go has slightly tighter constraints on `wasmimport` and `wasmexport` than TinyGo, which this commit works around:
- It disallows use of `*string` (as opposed to `string`)
- It disallows use of `*cm.List` (probably because of the `[any]`, but I'm not sure at all)

Signed-off-by: Till Schneidereit <till@tillschneidereit.net>

Author: tschneidereit