Add wasm build tags

[omerfirmak]

Fixes #61

[joshuacolvin0]

This would be helpful for https://github.com/OffchainLabs/nitro as well

[eljobe]

To add a little context to what @joshuacolvin0 has already mentioned.
We are currently maintaining our own fork of this repo here: https://github.com/OffchainLabs/fastcache essentially, just to add these build tags. It would be much nicer if we could just depend on the official repo, and occasionally pull in the latest version rather than having to maintain this dependency manually.

[makasim]

LGTM

[f41gh7]

LGTM

[f41gh7]

Thanks for contribution!