[feat] Implement sys_rand

[Golovanov399]

This resolves INT-2955.

sys_rand now:

• calls a new phantom instruction that adds a hint stream with some random numbers,
• then reads this many numbers from this hint stream to the required memory location.

The integration test is a simple one and just shows that the hashmap works somehow (it didn't use to because of "sys_rand unimplemented", but maybe we could also check the statistical randomness of our calls somehow.

[jonathanpwang]

Move integration test to rv32im/test.

Currently the phantom instruction is wrong: it needs to read len from memory address space 1.

Update ISA doc: it's easier to make a PR into #1291

[github-actions]

group	app.proof_time_ms	app.cycles	app.cells_used	leaf.proof_time_ms	leaf.cycles	leaf.cells_used
verify_fibair	(+31 [+1.4%]) 2,193	513,772	19,376,241	-	-	-
fibonacci_program	(+170 [+3.3%]) 5,265	1,500,095	51,485,080	-	-	-
regex_program	(+46 [+0.3%]) 15,059	4,139,840	163,831,459	-	-	-
ecrecover_program	(-11 [-0.4%]) 2,542	284,567	15,055,843	-	-	-

Commit: a5c71aa

Benchmark Workflow

[jonathanpwang]

LGTM