Typecheck does cross-platform typechecking of source code for all Go build

platforms.

The primary benefit is speed: a full Kubernetes cross-build takes 20 minutes

and >40GB of RAM, while this takes under 2 minutes and <8GB of RAM.

It uses Go's built-in parsing and typechecking libraries (go/parser and

go/types), which unfortunately are not what the go compiler uses. Occasional

mismatches will occur, but overall they correspond closely.

Failures can be ignored if they don't block the build:

Things go/types errors on that go build doesn't:

True errors (according to the spec):

These should be fixed whenever possible. Ignore if a fix isn't possible

or is in progress (e.g., vendored code).

- unused variables in closures

False errors:

These should be ignored and reported upstream if applicable.

- type checking mismatches between staging and generated types

Things go build fails on that we don't:

- CGo errors, including syntax and linker errors.