Can't use quasiquoter from libsyntax

[eholk]

The problem seems to be that the quasiquoter generates absolute paths, which means syntax::ext::qquote is not legal inside of libsyntax. We can probably fix this by generating paths relative to syntax::ext::qquote and requiring the user to be sure qquote is in scope when needed.

[eholk]

Nervmind, auto_serialize does it. Here's the magic:

// Transitional reexports so qquote can find the paths it is looking for
mod syntax {
    import ext;
    export ext;
    import parse;
    export parse;
}