cgo gets a rework: unmodified Go source to the compiler

The accepted proposal to have cmd/cgo hand unmodified Go source files to the compiler and go/types is the item to sit with, folding a long-standing preprocessing step back into the toolchain proper. It lands alongside a fresh, separate proposal to make cgo work without a C toolchain on the host, so the shape of cgo is clearly on the table right now. On the sidelines, a dev.to piece pushes back on the idea that function coloring and context propagation aren't a form of infection, which is a familiar argument but a reasonable one to reread.