initial support for branching: phi() function, track stack/locals for every pc-position

initial support for branching: phi() function, track stack/locals for every pc-position

added in preliminary support for branching.  It consisted of:

  - we have to track the state of the stack and localvars separately
    for each instruction-position

  - I introduced a phi() function that "merges" two expressions

  - whenever you branch _to_ an instruction, you have to "map" the
    merge() function over the target stack, passing the source stack
    as an argument (and likewise for locals).

Note that Phis are mutable (see the javadoc comment at the head of
"class Phi"), and you can't "collapse" nested Phi's, even though it's
tempting.

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