Function closures

[ahdinosaur]

Is your feature request related to a problem? Please describe.
You should be able to return a closure function.

Describe the solution you'd like

let
  equals: (a) => (b) => a == b
in
  equals(0)
    - 0

Implementation details
At the moment, we return functions as values, without the environment they "close over". A function value should now have an environment attached, which will be used when evaluating.

Describe alternatives you've considered

Additional context

[ahdinosaur]

oh dear...

notes:

• https://www.diegofreijo.com/blog/rlox-vm-a-lox-interpreter-in-rust-part-1/
• https://pulpdrew.com/tech/rlox/
• tree-walk interpreters:
  • https://github.com/UncleScientist/lox-ast
  • https://github.com/lffg/rs-lox/blob/main/tree-lox
  • https://github.com/veera-sivarajan/rena

[ahdinosaur]

first pass, using references and lifetimes, was terrible, and a dead-end: https://github.com/ahdinosaur/rimu/tree/function-closures

once i knew what to do, was straightforward: #62