Add support for nested comments

[madjar]

This should close #9468.

I removed the test stating that nested comments should not be implemented.

I had a little chicken-and-egg problem because a comment of the std contains "/*", and adding support for nested comment creates a backward incompatibility in that case, so I had to use a dirty hack to get stage1 and stage2 to compile. This part should be revert when this commit lands in a snapshot.

This is my first non-typo contribution, so I'm open to any comment.

[alexcrichton]

Would you mind squashing these commits into one? Also feel free to ping reviewers on the pull request when you update it because sadly github doesn't notify anyone on a push to the branch.

[madjar]

@alexcrichton I wasn't sure I should do that, thanks for the instructions.

@brson Fixed what you said.

[brson]

@madjar the main function of the test case you moved to run-pass needs to be pub because of the way all tests are concatenated together on the windows bots.

[madjar]

@brson Thanks, I wouldn't have guessed it myself. This should be fixed (and squashed).

[alexcrichton]

Hm, it looks like something mahy have gone awry, the test is still just fn main() {}

[madjar]

Silly me having troubles with git. This is now (really fixed).