Store source file contents in generated source maps.

This increases the size of the maps, but it also makes them substantially
more self-contained and thus more portable. And frankly? nobody much cares
if source map files are big.
Don't treat characters as names when adding mappings to a source map.

Omitting the optional `name` property has the added benefit of shortening
each encoded mapping (presumably because they no longer have to refer to
the names), so the overall size of the source map significantly decreases.

Closes #57.