Move Core.Compiler into Base
This is the first step in what I am hoping will eventually result in making
the compiler itself and upgradable stdlib. Over time, we've gained several
non-Base consumers of `Core.Compiler`, and we've reached a bit of a breaking
point where maintaining those downstream dependencies is getting more difficult
than the close coupling of Core.Compiler to the runtime is worth.
In this first step, I am moving Core.Compiler into Base, ending the duplication
of common data structure and generic functions between Core.Compiler and Base.
This split goes back quite far (although not all the way) to the early days of
Julia and predates the world-age mechanism.
The extant Base and Core.Compiler environments have some differences
(other than the duplication). I think the primary ones are (but I will add
more here if somebody points one out).
- `Core.Compiler` does not use `getproperty`
- `Core.Compiler` does not have extensible `==` equality
In this, I decided to retain the former by setting `getproperty = getfield`
for Core.Compiler itself (though of course not for the datatstructures shared
with Base). I don't think it's strictly necessary, but might as well.
For equality, I decided the easiest thing to do would be to try to merge
the equalities and see what happens. In general, Core.Compiler is relatively
restricted in the kinds of equality comparisons it can make, so I think it'll
work out fine, but we can revisit this.
This seems to be fully working and most of this is just moving code around.
I think most of that refactoring is independently useful, so I'll pull some
of it out into separate PRs to make this PR more manageable.
