Implement arbitrary loops within BMv2 actions

[jafingerhut]

BMv2 actions already implement conditional and unconditional forward or backward jump instructions, within the primitive actions of a single P4 action, so it should be possible to implement loops there with only moderate changes to the p4c BMv2 back end code. See:

• the documentation for _jump and _jump_if_zero primitive BMv2 actions in this document: https://github.com/p4lang/behavioral-model/blob/main/docs/JSON_format.md
• the implementation of arbitrary if statements within actions for possible inspiration: p4lang/p4c@c11bb8b

Things to think carefully about when working on this:

• we want break, continue, and exit statements in arbitrary places in a loop, including within if statements inside of a loop body, to work correctly.
• we want nested loops to work correctly.

Note that implementation arbitrary loops in the body of a control likely requires generalizing other parts of the BMv2 JSON file format / data model, probably with a lot of overlap with this project idea: #8