Rename fail! to panic!

https://github.com/rust-lang/rfcs/pull/221

The current terminology of "task failure" often causes problems when
writing or speaking about code. You often want to talk about the
possibility of an operation that returns a Result "failing", but cannot
because of the ambiguity with task failure. Instead, you have to speak
of "the failing case" or "when the operation does not succeed" or other
circumlocutions.

Likewise, we use a "Failure" header in rustdoc to describe when
operations may fail the task, but it would often be helpful to separate
out a section describing the "Err-producing" case.

We have been steadily moving away from task failure and toward Result as
an error-handling mechanism, so we should optimize our terminology
accordingly: Result-producing functions should be easy to describe.

To update your code, rename any call to `fail!` to `panic!` instead.
Assuming you have not created your own macro named `panic!`, this
will work on UNIX based systems:

    grep -lZR 'fail!' . | xargs -0 -l sed -i -e 's/fail!/panic!/g'

You can of course also do this by hand.

[breaking-change]
Update infrastructure for fail -> panic

This includes updating the language items and marking what needs to
change after a snapshot.

If you do not use the standard library, the language items you need to
implement have changed. For example:

```rust
 #[lang = "fail_fmt"] fn fail_fmt() -> ! { loop {} }
```

is now

```rust
 #[lang = "panic_fmt"] fn panic_fmt() -> ! { loop {} }
```

Related, lesser-implemented language items `fail` and
`fail_bounds_check` have become `panic` and `panic_bounds_check`, as
well. These are implemented by `libcore`, so it is unlikely (though
possible!) that these two renamings will affect you.

[breaking-change]
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: 

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