Fix typography of "C." (spacing)
[expr.prim.general] Use code font for ~.
[cpp] Fix font of # in non-directive.
[gram] Don't convert one space to two after a `.` or `!` in the grammar.
These do not mark the end of a sentence.
Use a consistent size and position for all tildes.
Replace all '\sim's with '\~'s, to give more consistent formatting for ~s.
Merge pull request #348 from tkoeppe/typography1

Fix typography of "C." (spacing)