* flower/polynomial.cc (operator +): optimize += operator. No

* flower/polynomial.cc (operator +): optimize += operator. No
copying.

* lily/source-file.cc (get_line): use binary search to determine
line number. This kills another quadratic time-complexity term.

* lily/include/source-file.hh (class Source_file): add newline_locations

* flower/include/array.icc (binary_search_bounds): new function.