Add test/typecheck, a fast typecheck for all build platforms.

Most of the time spent compiling is spent optimizing and linking
binary code. Most errors occur at the syntax or semantic (type) layers.
Go's compiler is importable as a normal package, so we can do fast
syntax and type checking for the 10 platforms we build on.

This currently takes ~6 minutes of CPU time (parallelized).

This makes presubmit cross builds superfluous, since it should catch
most cross-build breaks (generally Unix and 64-bit assumptions).

Example output:

$ time go run test/typecheck/main.go
type-checking:  linux/amd64, windows/386, darwin/amd64, linux/arm, linux/386, windows/amd64, linux/arm64, linux/ppc64le, linux/s390x, darwin/386
ERROR(windows/amd64) pkg/proxy/ipvs/proxier.go:1708:27: ENXIO not declared by package unix
ERROR(windows/386) pkg/proxy/ipvs/proxier.go:1708:27: ENXIO not declared by package unix

real	0m45.083s
user	6m15.504s
sys	1m14.000s

test/BUILD

@@ -21,6 +21,7 @@ filegroup(
         "//test/list:all-srcs",
         "//test/soak/cauldron:all-srcs",
         "//test/soak/serve_hostnames:all-srcs",
+        "//test/typecheck:all-srcs",
         "//test/utils:all-srcs",
     ],
     tags = ["automanaged"],