Auto merge of #38066 - bluss:string-slice-error, r=sfackler

Auto merge of #38066 - bluss:string-slice-error, r=sfackler

Use more specific panic message for &str slicing errors

Separate out of bounds errors from character boundary errors, and print
more details for character boundary errors.

It reports the first error it finds in:

1. begin out of bounds
2. end out of bounds
3. begin <= end violated
3. begin not char boundary
5. end not char boundary.

Example:

    &"abcαβγ"[..4]

    thread 'str::test_slice_fail_boundary_1' panicked at 'byte index 4 is not
    a char boundary; it is inside 'α' (bytes 3..5) of `abcαβγ`'

Fixes #38052