`ltk_ritobin`: refactor `open_brace_span` into a more abstract API

[Crauzer]

#167 (comment)

Proposed:

• ChildRange::tokens(cst: &Cst) -> impl Iterator<Item = Token>
• ChildRange::nodes(cst: &Cst) -> impl Iterator<Item = Node>
• ChildRange::first_token_of_kind(kind: TokenKind, cst: &Cst) -> Option<Token>
• ChildRange::first_node_of_kind(kind: TreeKind, cst: &Cst) -> Option<Node>