Rust libsyntax ast builder.

Example

Here is a simple example to build the syntax::ast::Expr that represents adding two numbers together:

```rust extern crate aster; extern crate syntax;

fn main() { let builder = AstBuilder::new();

let expr = builder.expr()
    .add().u32(1).u32(2);

// prints `1 + 2`.
println!("{}", syntax::pprint::expr_to_string(&expr));

} ```