and actually not until `builtins.mergeio`.
I would have liked to delay typechecking in IOSource.hs altogether, but
it being forced somewhere that was not immediately obvious to me.
This shouldn't impact anyone who already has `.base` or some other copy
of the io primitives, and the upshot is saving some time on transcripts.
`builtins.merge` will give you everything that's in `Unison.Builtin` and
`Unison.Builtin.Decls`, `builtins.mergeio` will give you those plus the
stuff in `Unison.Runtime.IOSource`.
