Precede infix "<<" with "op".
Move bitTheory into src/num/extra_theories and move evaluations theorems in wordsLib further up the build sequence.

Includes some tidying-up work.

This closes issue #120. Evaluation for "toNum" and "toString" can now be enabled by loading up stringLib (or, more precisely, ASCIInumbersLib), without the need to load up wordsLib.

This check-in can be viewed as finishing off the (many) loose ends left over from work on issue #70.