fix a bug in bitv where the undefined bits in small/big bitv values could cause operations to return incorrect results.

Require comma separators for all expression types except the plain block

API is (for now) mostly by value, there are options to use it by
reference if you like.  Hash and equality functions must be pure
and by reference (forward looking to the day when something
like send_map becomes the standard map).

Rewrote bitv as a class that uses a 32-bit int as its representation
for bit vectors of 32 bits or less, and a vector (the old representation)
otherwise. I didn't benchmark very much, but a bit of informal benchmarking
suggested this is a win.

Closes #2341

convert doc-attributes to doc-comments using ./src/etc/sugarise-doc-comments.py (and manually tweaking) - for issue #2498

Changed a while loop into a for loop in std::bitv::equal. Yay!

Instead, use vec::to_mut/from_mut to transform vectors in place as
needed.