Added support for a `\0` escape sequence.

This commit adds support for `\0` escapes in character and string literals.

Since `\0` is equivalent to `\x00`, this is a direct translation to the latter
escape sequence. Future builds will be able to compile using `\0` directly.

Also updated the grammar specification and added a test for NUL characters.
auto merge of #9248 : DanRosenwasser/rust/master, r=alexcrichton

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