dan: "I probably wouldn't expect those mappings to cause much
trouble at all for legitimate users at the end of the day"

shivaram: [concurs]

'm' skeletonizes to 'rn' (but is exempted by the isBoring check),
but the fullwidth 'ｍ' does not skeletonize to anything. The root cause
of this is the (still unexplained) patchiness of the skeleton mapping
for fullwidth -> standard-width Latin characters; the fix is to perform
width mapping first, before either skeletonization or isBoring.

Fix spurious bidi rule violations in casefolding channel names
by stripping the # before starting the casefolding.

This matches the behavior of inspircd at the very least.

Previously, the comment above that section claimed ':' should be
disallowed, but the code didn't do so.

I also simplified the code a little bit and added tests.