macro-ize `fun leaf(args, ...)`

[OxSon]

per Matthew's feedback

At the point where you worry about performance, the leaf function may need to be converted into a macro that constructs a vector more directly. Right now, leaf will build up a list to receive its arguments, and then have to apply the vec function to that list. And then the vec function will also receive the arguments in a fresh list and apply the #{vector-immutable} function. It's possible that inlining at the Racket or Chez Scheme level works better than I think, but I don't think the intermediate list allocations will be optimized away.

• [ ] in utils.rhm, don't wrap racket's vector->immutable-vector in a rhombus function, just export and rename to vec
• [x] make leaf a macro instead of a function

[OxSon]

done