Split out bitvector.h to a submodule

[randombit]

The motivation here (and also #4456 #4457) is to reduce the size of utils which is a mandatory-to-compile module, in particular with a mind that even headers that are completely unused are compiled into the amalgamation source. With the meta-goal that with some work and luck it might be possible to reduce amalgamation-related miscompilations (#4454 #3097) to something small enough to make a viable bug report. But even if not, it reduces the size of the amalgamation for the common case (common for amalgamation that is) of a build with only selected modules enabled, since bitvector.h is only used for CMCE and that's a relatively niche algorithm.

Splitting bitvector.h to a submodule reduces the size of the minimized amalgamation (--amalgamation --mini) from 13480 lines to 12059 lines.

[coveralls]

coverage: 91.266% (+0.002%) from 91.264%
when pulling 0514ca2 on jack/split-bitvector-to-submodule
into 2616446 on master.

[reneme]

Makes sense, thanks. We had considered that in the context of the SuperH ICE as well.