Multiple lookups, simpler rec environment

[gabriel-barrett]

This PR simplifies recursive environments. Now, instead of it taking an environment itself, the tag for the symbol is changed to denote a recursive variable. Although we can't get mutual recursion like this, there's a possible fix:

(letrec ((f A) (g B))
  C)

can be transformed into

(letrec ((f (letrec ((g B)) A)))
  (letrec ((g B)) C))

while reducing, which will achieve mutual recursion. This will, however, add some overhead to non-mutually recursive functions. But again, this can be mitigated if we take the memoset approach to letrec.

Now, because of the simplification, it is possible to do 3 lookups per step without adding any slots. This greatly increases performance. (fib 20) goes from 613 iterations to 390, almost half the number of iterations

[huitseeker]

The better use of tags is great, I left a question, mostly geared at maintenability.

[huitseeker]

1. How much more expensive would it be to add a proper case in the environment?
2. If the answer is "quite a bit", could we document this special meaning in the definition of the Thunk variant in the enum declaration?

I also wouldn't mind removing the mention of "hack" here, and renaming the "Thunk" variant to ThunkOrRecSym (or something more evocative).

[gabriel-barrett]

@huitseeker not expensive at all, I think it would just add 1 constraint and 1 auxiliary. I just didn't do it because I'd have to add a bunch of cases in a bunch of files. I think I will add a new tag "FrozenVar", since it is a frozen computation that the variable refers to. I'll replace the comment as well

[huitseeker]

The specialized RecVar looks great, the cycle counts going down also looks nice. I'd love for @arthurpaulino and / or @porcuquine to have quick look, but this LGTM

[arthurpaulino]

My review is summarized in this Zulip thread. The first bullet point is solved with the specialized RecVar tag.

On the second bullet point, I'd like us to, at least, plan for a solution for mutual recursion which makes us all content, as the syntactical manipulation mentioned in the PR description doesn't satisfy me completely (at least not yet).

[gabriel-barrett]

!gpu-benchmark

[github-actions]

Benchmark for 753bf71

Click to view benchmark

Test	Base	PR	%
LEM Fibonacci Prove - rc = 100/fib/num-100	2.4±0.00s	1732.1±1.67ms	-27.83%
LEM Fibonacci Prove - rc = 100/fib/num-200	4.6±0.01s	3.3±0.00s	-28.26%
LEM Fibonacci Prove - rc = 600/fib/num-100	1983.5±4.36ms	1956.5±5.37ms	-1.36%
LEM Fibonacci Prove - rc = 600/fib/num-200	4.5±0.01s	3.3±0.01s	-26.67%