Add compile-time / macros that can do loops and conditionals

[jafingerhut]

Personnel

• Owner: todo
• Supporters: @jafingerhut

Design

• Document:

Implementation

• p4-spec:
• p4c:

Process

• LDWG discussed:
• LDWG approved:
• Merged into p4-spec:
• Merged into p4c:

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The issue of adding execution-time loops to P4 is tracked separately, in issue #1325

Earlier discussion that led to creation of this issue was done for this more narrow issue: #1261