Exorcise fmt from the library packages

[Ian Denhardt]

We discussed this a while back on matrix: I'm doing some stuff where I'm compiling Go to webassembly and using capnp to communicate between a browser and a server. I'm using TinyGo to try to keep executable sizes down, but the fmt package in particular pulls in a lot of stuff. There's no good reason why we should need to pull it in as a dependency; I'd like to avoid its use in the library parts of this (tests and code generator are fine) so we can output smaller binaries where it matters.