`rexpr` macro enhancement

The current algae_grammar::rexpr macro is currently not really safe or nice to use. It does some basic checks (tracking that variables are accessible to prevent runtime panic), but it the structure makes no sense the resulting error is somewhat unreadable.

enhancement

A solution would be to do better parsing (try to find types, check for soundness). Ultimately the resulting SubOp would carry a lot more context information which should make token emission safer and faster.