Better way to inject core macros

[catamorphism]

From a FIXME in src/librustcsyntax/ext/expand.rs (just before the core_macros() function):

"FIXME: this is a terrible kludge to inject some macros into the default
compilation environment. When the macro-definition system is substantially
more mature, these should move from here, into a compiled part of libcore
at very least."

The macros in question are #error, #warn, #info, and #debug. The issue is to do something less kludgy ;-)

[msullivan]

Issue #3114 seems like the approach we want to fix this. Changing the milestone to 3 to match #3114.

[msullivan]

Nothing new.

[catamorphism]

Not 1.0, low