visitor: Handle specified line ranges in visit_stmt

visitor: Handle specified line ranges in visit_stmt

This commit adds a very rough implementation of handling the specified
line ranges in `config.file_lines_map` for statements. It reformats a
statement if its span is fully contained in the set of lines specified
for the file.

The implementation here is intended as a proof of concept, and
demonstration that the machinery added in the preceding commits is
functional. A final implementation would likely hook in via the
`Rewrite` trait.

Refs #434