Bracket sizing should be based on levels of nesting

Currently, the outermost bracket pair is always the largest possible size (125% of normal text height), and they get smaller from there. The means that even when you only have one pair of brackets on a line, they are the largest they can be.

Instead, sizing should count from the innermost pair. That means if there's only one pair of brackets, they should be regular-sized. Only if there are nested brackets will they start to grow larger. That also means we can make deeply nested brackets even bigger, because the extra space is **only** used when it's really needed because the programmer nests brackets deeply.

Unfortunately, this does mean the entire file must be rescanned to change parenthesis sizing, and changes later on can affect much early portions of the file. It doesn't fit so well with the existing forward-only syntax highlighting scheme that CodeMirror uses. Making this work is an interesting open question.