Split megaton-hammer's RT0 stuff

[roblabla]

Right now, megaton-hammer is a bit of a mess: it contains both stuff meant for stdlib consuption, stuff meant for user consuption, and stuff meant for "overlay library" consuption. The abstraction boundaries are unclear, and implementation details leak all over the place.

So let's take a look at one particular issue: megaton_hammer::loader. This module is, sadly, public, even though it should really be internal. This is mostly due to one issue: std needs access to megaton_hammer::loader::Logger.