Fix interpreter to work with new interface

[Alasdair]

Notably, effect_request type simplified to:

and effect_request =
  | Read_reg of string * (value -> state -> frame)
  | Write_reg of string * value * (unit -> state -> frame)
  | Outcome of id * value list * (return_value -> tannot exp Monad.t)

[github-actions]

Test Results

   13 files     27 suites   0s ⏱️
  857 tests   857 ✅ 0 💤 0 ❌
3 740 runs  3 740 ✅ 0 💤 0 ❌

Results for commit c797c84.