Add trait Parse and Parse::parse to use with each types.

With code 
```rust
let parsed = parse(code, path, Mode::Exec)
```

The result `parsed` is always `ast::ModModule`. But it currently returns `ast::Mod::Module`.

suggestion:
```rust
let parsed = ast::ModModule::parse(code, path)
```

previous `parse_*` functions can remains for CPython friendly API