Remove unneeded exports in syntax::parse::parser

[catamorphism]

parser re-exports parse::parse_from_source_str because the #ast macro uses it. With pub/priv, the #ast code could just call it directly and this extra declaration could be removed. I tried to fix the exports, but ran into problems.

[catamorphism]

Not critical for 0.6, de-milestoning

[catamorphism]

Seems to be fixed now.