tinygo support

[bradleypeabody]

This PR makes fwd compile and test correctly with TinyGo. After some review, I think this change makes more sense than using the not-unsafe version under TinyGo - since unnecessary allocations are particularly problematic in microcontrollers environments.

The actual change is very minor: just one cast from int to uintptr. But I had to copy the function and make a TinyGo-specific version.

For reasons explained here, reflect.SliceHeader is defined slightly differently in TinyGo vs Go.

The tests now pass in TinyGo with:

GO111MODULE=off tinygo test .

Or if you have Docker installed but not TinyGo, you can do:

docker run --rm -v $(pwd):/src -e GO111MODULE=off tinygo/tinygo:latest bash -c 'cd /src/ && tinygo test .'

[bradleypeabody]

Also just FYI, I can think of a few other microcontroller-specific changes that should probably be done. E.g. the default sizes should probably be lowered for TinyGo (I'm thinking more like 64 or maybe 256 bytes instead of 2k). And allowing the caller to explicitly provide a read or write buffer, so they could do things like use an array allocated at package level which TinyGo will put in the BSS instead of doing a heap allocation (helps avoid memory fragmentation).

But rather than getting into all of that here in this PR, I think it will be most productive to just start with this PR and then perform whatever TinyGo-specific tuning as part of the changes to msgp and then wrap back around and do a separate PR for fwd if needed.

[philhofer]

Thanks