Pipes now have a ghost that auto-expands to the end #hard

`feature.auto-expanding-parens` #262 removed `'|'` from `CharCmds` and never
added anything back, so pipes were just `VanillaSymbol`s.

Note that transient ghosts pretty much resolves #81: typing nested
absolute values is still more annoying than typing nested parens,
because inside the one-sided absolute value, typing the first pipe still
closes the absolute value, but now that the ghost has been solidified
you can go Left and just type the inner absolute value; whereas
originally, there was no one-sided/ghost state, you just couldn't type
an inner absolute value at the end.

Also misc. delimiters-related fixes.