chore: forget cached MultiFrame witnesses after they're used

[arthurpaulino]

The following profiles are from calling lurk with --rc 100 and --limit 10000 and then proving the following (infinite, but capped) computation:

(letrec ((loop (lambda () (loop)))) (loop))

Before:

After:

Note: the memory peak wasn't affected on my weak machine. But if the computation is long enough (so it takes longer to generate the witnesses for all multiframes) and there is a GPU available (so each proving step is faster), used witnesses from earlier multiframes will start being cleared out before new witnesses from later frames are created.

[arthurpaulino]

!gpu-benchmark

[github-actions]

Benchmark for b694c16

Click to view benchmark

Test	Base	PR	%
LEM Fibonacci Prove - rc = 100/fib/num-100	1463.2±8.39ms	1456.3±9.52ms	-0.47%
LEM Fibonacci Prove - rc = 100/fib/num-200	2.8±0.01s	2.8±0.01s	0.00%
LEM Fibonacci Prove - rc = 600/fib/num-100	1838.7±3.28ms	1838.9±12.43ms	+0.01%
LEM Fibonacci Prove - rc = 600/fib/num-200	3.0±0.02s	3.0±0.02s	0.00%