indexing

"Sound unchecked indexing" in Rust using "generativity" (branding by unique lifetime parameter).

This is a partial fork of https://github.com/bluss/indexing, with most of the code directly inspired from such.

This fork is immutable only, doesn't offer pointer indices/ranges, and doesn't allow manipulating the indices/ranges without a reference to the container. In exchange for these limitations, it works for fully sound fully unchecked indexing of Rust's UTF-8 string types.