Introduce InnerSpan abstraction

Introduce InnerSpan abstraction

This should be used when trying to get at subsets of a larger span,
especially when the larger span is not available in the code attempting
to work with those subsets (especially common in the fmt_macros crate).

This is usually a good replacement for (BytePos, BytePos) and (usize,
usize) tuples.

This commit also removes from_inner_byte_pos, since it took usize
arguments, which is error prone.