This switches fish_indent from parsing with parse_tree
to the new ast.
This is the most difficult transition because the new ast retains less
lexical information than the old parse tree. The strategy is:
1. Use parse_util_compute_indents to compute indenting for each token.
2. Compute the "gap text" between the text of significant tokens. This
contains whitespace, comments, etc.
3. "Fix up" the gap text while leaving the significant tokens alone.
This isn't quite the old-style test, but it checks some of the line
continuation stuff.
Note that littlecheck ignores leading whitespace, so testing the
actual indentation requires some more effort.
