1368: Store referece instead of full token tree in tokenbuffer r=matklad a=edwin0cheng
This PR try to minimize the memory allocation in converting `SyntaxNode` to `TokenTree` by using reference isnteead of full token tree in `TokenBuffer`.
Note that the final goal is replace `TokenTree` with TokenBuffer such that there is no conversion between them.
Co-authored-by: Edwin Cheng <edwin0cheng@gmail.com>