BREAKING CHANGE: (or perhaps, *bugfix*)

In #![no_std] applications, the following calls to `panic!` used
to behave differently; they now behave the same.

Old behavior:

    panic!("{{");   // panics with "{{"
    panic!("{{",);  // panics with "{"

New behavior:

    panic!("{{");   // panics with "{{"
    panic!("{{",);  // panics with "{{"

This only affects calls to `panic!` (and by proxy `assert`
and `debug_assert`) with a single string literal followed by
a trailing comma, and only in `#![no_std]` applications.

Most notably this changes 'syntax::ext::base::get_single_str_from_tts'
to accept a trailing comma, and revises the documentation so that this
aspect is not surprising. I made this change under the understanding
that this crate is private rustc implementation detail (I hope this is
correct!). After reviewing all call sites, I believe the revised
semantics are closer to the intended spirit of the function.

include! and the pretty test do not mix