[WIP] Add example implementation for bounded foreach-style loops

[asl]

For #4120 and p4lang/p4-spec#1261

This is an example implementation for foreach-style bounded loops. It contains no tests (as they are written for dowstream backend) and some sema checks are certainly lacking, but still shows the intention. One thing that is currently missed is the use-def changes, I have not ported them to upstream, but will re-add shortly. Certainly changes to other passes might be required (e.g. copyprop).

The PR has some tricks how to handle loop index variable in order to play nicely with the existing code.

Examples of supported code:

action a0(bit<8> x, bit<8> y) {
        // Reduce
        bit<64> idx = -1;
        foreach (bit<8> i in 0 .. x) {
            foreach (bit<8> j in i .. y) {
                idx = foo(j);
                if (idx == -1)
                    break;
            }
        }
    }

    action a1(bit<8> x, bit<8> y) {
        // Reduce
        bit<64> idx = -1;
        foreach (bit<8> i in 0 .. x) {
            foreach (bit<8> j in 0 .. y) {
                idx = foo(j);
                if (idx == -1) {
                    continue;
                }
                idx = foo(i);
            }
        }
    }

[asl]

Tagging @jafingerhut and @ChrisDodd

[asl]

This is structured in a way to simplify review. It could be merged into the token list above

[asl]

Another part missed here from downstream: type inference check that types of loop index variable is compatible with the range.

[fruffy]

Are the changes here merged into #4562? Can this be closed?

[ChrisDodd]

Are the changes here merged into #4562? Can this be closed?

There were minor syntax changes (for instead of a new foreach keyword), but I believe this can be closed.