Renames StringAnnotation to Span, and moves `in` method out of Span so that Span isn't tied exclusively to Strings. The `in` method is now available via an implicit value class, so the overhead of using `in` should not change at all.
Adds a type alias `StringSlab[...] = Slab[String, Span, ...]`. Note that this needs to be a type alias, not a trait, otherwise the Slab type parameters will have to get even more complicated to ensure that a StringSlab subclass of Slab always returns StringSlabs from all the methods that currently return Slabs.
Merge pull request #21 from bethard/master

Renames StringAnnotation to Span, adds StringSlab alias