Add Handles

[Joeoc2001]

The Naga project uses a Handle-Arena approach to AST management rather than a Box-Ownership model. I'm fond of this for a few reasons, including performance and ease of AST mutation (especially with a visitor pattern, where you might first visit immutably and collect handles and then perform modifications).

This PR adds the option to generate an Arena-based grammar. It doesn't change any public signatures of existing generated code, but upon declaration of a grammar with a least one Handle<T> field the rust_sitter::language macro additionally requires a rust_sitter::arena type somewhere in the declaration:

#[rust_sitter::grammar("arithmetic")]
pub mod grammar {
    use rust_sitter::Handle;

    #[rust_sitter::arena]
    #[derive(Default, Debug)]
    pub struct MyArena;

    #[rust_sitter::language]
    #[derive(PartialEq, Eq, Debug)]
    pub enum Expression {
        Number(#[rust_sitter::leaf(pattern = r"\d+", transform = |v| v.parse().unwrap())] i32),
        #[rust_sitter::prec_left(1)]
        Sub(
            Handle<Expression>,
            #[rust_sitter::leaf(text = "-")] (),
            Handle<Expression>,
        ),
        #[rust_sitter::prec_left(2)]
        Mul(
            Handle<Expression>,
            #[rust_sitter::leaf(text = "*")] (),
            Handle<Expression>,
        ),
    }

    #[rust_sitter::extra]
    struct Whitespace {
        #[rust_sitter::leaf(pattern = r"\s")]
        _whitespace: (),
    }
}

Then, parse returns both the root Expression and an instance of MyArena which the handles point in to.

As a note, while this improves the performance of AST transformations it currently this doesn't change the performance of parsing (about a 1-2% speedup). I'd anticipate that a preliminary traversal of the tree for an arena pre-allocation would unlock more performance, so if that's a requirement to get this merged then let me know and I'll give it a go. Otherwise, as it stands, this allows for a new paradigm with associated performance characteristics without changing much else.

[shadaj]

Hey! Just wanted to ack this PR, will try to look through it carefully soon. This looks broadly to be in the right direction; would be really awesome to explore a prototype of what incremental updates could look like just to make sure this does indeed allow for that.

[Joeoc2001]

Thanks for the ack! I've added an example where arithmetic has had one Boxs replaced with a Handle - let me know if that wasn't what you were looking for