Use Span to represent source locations

[ishitatsuyuki]

Span seems to be a quite common thing in parsers, and it's also very useful when it comes to source map (#98, sourcemaps uses line and column instead of byte position).

Instead of reimplementing these functionality in-house, there seems to be a good crate, codespan that is used by the Gluon scripting language. Let's integrate it (and give them a shout out)!