Added `slice_chars_from` and `slice_chars_to` methods to StrSlice

They are occasional useful, and not having them is inconsistent to the byte-indice slice methods.

`slice_chars`, `slice_chars_from` and `slice_chars_to` are now implemented in terms of a common `slice_chars_common` function.

I also made the section headers in `str.rs` more visible, as they are they where to small to be really useful.