[bug#28586] go-build-system and Syncthing

[Leo Famulari]

These patches add a go-build-system, a package for Syncthing, and
Syncthing's dependencies.

The patch for Syncthing and its dependencies is squashed to make review
more convenient (there are dozens of packages). I can split this up into
individual commits before pushing.

There are a handful of XXX comments in (guix build go-build-system).
Advice welcome :)

Also, this build system does not attempt cross-compilation, earning
another XXX in (guix build-system go).

I've skipped a few package test suites in (gnu packages syncthing).
However, everything works for me, and Syncthing passes its test suite,
at least.

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