RFC: Tuple indexing

[Rendered view](https://github.com/P1start/rfcs/blob/tuple-accessors/active/0000-tuple-accessors.md)
@glaebhoerl I would imagine that float literals become involved because this RFC would probably be implemented by modifying the parser, whereas float literals are a type of token output by the lexer (since the lexer typically doesn't have much of a notion of context).