gc.c: Add GC.get_parameters and .set_parameters
These methods are for inspecting and modifying MRI's GC parameters. It may be
useful for realtime parameter tuning with GC.stat, user requests and so on.
This work is done by Tomohiro Moro (@slightair) and me (@makimoto).
GH issue: https://github.com/ruby/ruby/pull/1572
We're trying this patch with our environments and report here.