merlin.core submodule should probably be called merlin.utils

[karlhigley]

Once the worker submodule moves to IO, everything that's left in merlin.core is really utils, so we should call it utils instead. utils isn't great (since every utils package eventually turns into a junk drawer), but it's still clearer than core, which is a great name for the library but a terrible name for a submodule within a library of the same name.