Implement Copy trait on AstNode

[WindSoilder]

I noted there is a to do comment about AstNode:

// TODO: All nodes with Vec<...> should be moved to their own ID (like BlockId) to allow Copy trait

This pr is going to implement it.

To be honest I'm not really sure if it's intended, because it introduces some indirection. But I think it addresses comment from #68 (comment) to introduce some helper functions to Compiler, so we can use it in a better way.

[kubouch]

Seems good. Size of AstNode is unchanged (64 bytes) but it seems to accelerate the parsing a bit (eg. combine_compile_iter1000 is now up to 1ms faster on my machine).

One possible future optimization could be to store those IDs now living in the separate Vecs into either the ast_nodes, or an equivalent of extra_data from the Zig parser, and refer to them as NodeId + length, for example.