refactor(ast): make AstBuilder Copy (#3602)

`AstBuilder` only contains a shared ref `&Allocator`. So make it `Copy` and pass around `AstBuilder<'a>` instead of `&AstBuilder<'a>` - 1 less level of indirection.

Hopefully this will have little effect on benchmarks as all `AstBuilder`'s methods are marked `#[inline]`, but this change will minimize impact if compiler decides not to inline for some reason.