Set mostly-correct endCol on numbers, single-quoted strings and subscripts.

Before, endCol of number/string literals was identical to startCol;
 endCol of subscripts was the end of their slice node. This caused
 highlighting glitches when trying to position things after them.

Unaddressed:
 - Triple-quoted (particularly multiline) strings. This gets length 2
    (inc. quotes) or does nothing, which is no worse than at present.
   Such strings are very unusual inside expressions where we care about
   the length.

 - Space between the end of a slice and the closing bracket of a
   subscript: `a = b[ 2 ]`. Recommended against by PEP-8, but happens.

This should fix most remaining examples of bug 373850.
CCBUG: 373850