Draft: Wasm/SCORU: Allow limit number of computations for WASM PVM

Owner: @ole.kruger

Context

Prerequisite for #3548.

This MR replaces Lwt in Lib_webassembly with a new monad Action. Values of type Action.t are computations that can be run to produce an lwt promise in a limited number of bind invocations:

val run : ?max_num_steps:int -> 'a t -> 'a Lwt.t

The plan is to use this in Wasm_pvm to prevent that compute_step executes in too many steps.

Manually testing the MR

CI

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR