Move to the PDA2 style of interpretation

Now balanced paths in PDA2 are

  Context x State x Code

We must provide a new FlowAnalysis struct:

    flow :  Code
         -> Context State Configuration
         -> [Values [SetOf State x Code] Configuration]
    flow-ctx :  Code
             -> Context State ContextState Configuration
             -> [Values [SetOf Context] ContextState Configuration]
    flow-across :  Context [SetOf Context] ContextState Configuration
                -> [Values [SetOf State x Code] ContextState Configuration]
    initial-work-set : [SetOf Context x State x Code]
    initial-configuration : Configuration
    initial-ctx-state : ContextState
Remove stack and input stream from abstract states

The stack is maintained by the context and the input stream is not
important so let's not complicate the gte? predicate
Write a faster ctx-gte? and provide it directly

Generics are slow so let's implement this stuff directly. I'm also
using the fast term equal predicate.
Provide abstract-state-gte? directly

We cannot rely on generics to be fast enough so we must provide the
lattice function to our users