Closes #3302

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

Also adds proper checking for cont/break being inside a loop.

Closes #1854
Issue #1619

And remove spurious fails/unreachable() calls.

This mod only had two functions, all of whose users have been changed
to use the uint module.

Closes #1931

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

Closes #1849