auto merge of #17894 : steveklabnik/rust/fail_to_panic, r=aturon
This in-progress PR implements https://github.com/rust-lang/rust/issues/17489.
I made the code changes in this commit, next is to go through alllllllll the documentation and fix various things.
- Rename column headings as appropriate, `# Panics` for panic conditions and `# Errors` for `Result`s.
- clean up usage of words like 'fail' in error messages
Anything else to add to the list, @aturon ? I think I should leave the actual functions with names like `slice_or_fail` alone, since you'll get to those in your conventions work?
I'm submitting just the code bits now so that we can see it separately, and I also don't want to have to keep re-building rust over and over again if I don't have to :wink: