Added support for a \0 escape sequence

[DanielRosenwasser]

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.

This pull request is meant to address Issue #9143 .

[brson]

This is a language change. Let's discuss before landing.

[alexcrichton]

Hmm, I think that there's two things this should get before landing

1. The documentation in doc/manual.md should get updated
2. There should be a test to ensure the right behavior shows up.

[DanielRosenwasser]

@alexcrichton

1. Assuming you meant doc/tutorial.md, I chose not to make a change in the tutorial because I saw that \\, \xNN, \uNNNN and \UNNNNNNNN were not documented there. It's implied that the tutorial is not trying to be comprehensive on escape sequences, so I think it might be best to omit it.
2. I agree, a test would probably be in order, but I will probably hold off on this until it's clear that making \0 an escape character is appropriate for the language.

[alexcrichton]

Oh sorry about that, I meant doc/rust.md, I saw some documentation for the other escape sequences and this should probably go in with those.

We also agreed in the meeting today that this is fine to include, so no worries there (probably should have mentioned that too...).

[DanielRosenwasser]

Gotcha - well I think I've addressed some of the concerns. How's it looking now?

[alexcrichton]

Code looks good to me! Would you mind rebasing/squashing into one commit? That should also help get rid of the two merge commits that seem to have crept in.

[DanielRosenwasser]

You wouldn't know it, but I think I just learned a lot about git.

[alexcrichton]

Thanks!