control flow: loops

[quinn-dougherty]

Allow the user to write conditionals and loops in squiggle

EDIT: this is the loops issue. See 174 for if.

[Hazelfire]

Should this be like... functional loops and conditionals? I feel like it would be useful for Squiggle code to have guaranteed termination

[OAGr]

I'm pretty unsure here. I think conditionals are probably needed. Loops with control flow are more controversial; I'm not very sure here. We could try pushing loops off for a while.

Control flow is tricky to reason about, because we need to decide how to treat distributions in different cases.

[Hazelfire]

Agreed, I think loops and conditionals come up surprisingly little in practical models. I don't believe this is a priority

[quinn-dougherty]

Please discuss if on #174 -- this issue is for the loop question.