Move GC to its own directory and library

[d-netto]

With the ongoing MMTk effort, we are aiming to make the GC more modular and allow users to specify at build time different GC implementations they want for Julia.

This is the first step in this multi-PR effort.

For now, makes the GC as a static library to keep things as simple as possible.

[d-netto]

Bump.

[fingolfin]

Shouldn't src/julia_gcext.h also be moved? Basically everything it defines is implemented in src/gc.c

[d-netto]

Shouldn't src/julia_gcext.h also be moved? Basically everything it defines is implemented in src/gc.c

Possibly.

[d-netto]

Final bump. Will merge on Thursday if no objections.

[d-netto]

@nanosoldier runbenchmarks(!"scalar", vs=":master")

[nanosoldier]

Your benchmark job has completed - possible performance regressions were detected. A full report can be found here.