Rename fail! to panic!

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: 

Listing all the bits so I can remember as I go:

- [x] compiler-rt
- [x] compiletest
- [x] doc
- [x] driver
- [x] etc
- [x] grammar
- [x] jemalloc
- [x] liballoc
- [x] libarena
- [x] libbacktrace
- [x] libcollections
- [x] libcore
- [x] libcoretest
- [x] libdebug
- [x] libflate
- [x] libfmt_macros
- [x] libfourcc
- [x] libgetopts
- [x] libglob
- [x] libgraphviz
- [x] libgreen
- [x] libhexfloat
- [x] liblibc
- [x] liblog
- [x] libnative
- [x] libnum
- [x] librand
- [x] librbml
- [x] libregex
- [x] libregex_macros
- [x] librlibc
- [x] librustc
- [x] librustc_back
- [x] librustc_llvm
- [x] librustdoc
- [x] librustrt
- [x] libsemver
- [x] libserialize
- [x] libstd
- [x] libsync
- [x] libsyntax
- [x] libterm
- [x] libtest
- [x] libtime
- [x] libunicode
- [x] liburl
- [x] libuuid
- [x] llvm
- [x] rt
- [x] test