Experiment with removing unsafe slice indexing

[hsivonen]

Back when I made some slice indexing use unsafe, the compiler wasn't able to elide the bound checks on its own. However, when I made utf8_iter and pasted code into Compiler Explorer, I noticed that the compiler had learned to elide the bound checks.

Experiment with which instances of unsafe manual bound check elision would now work as safe code without a performance penalty.

[hsivonen]

One way of consolidating unsafe would be to use core::hint::assert_unchecked in the ascii_copy macro to tell the optimizer that consumed is within slice bounds and non_ascii is non-ASCII and then to remove unsafe in the callers.