Fixes N/A.

Summary/Motivation:

Eliminate duplicate walking of the expression tree when processing unhashable index values.

Changes proposed in this PR:

This PR creates an extension to evaluate_expression() with the 'constant' argument. When constant=True, this raises exceptions that reflect the key error messages that we want to return: (a) variables and (b) mutable parameters.

This change doesn't directly impact Pyomo performance in most cases, but the following script illustrates a case where the new code is twice as fast as the master branch:

from pyomo.environ import *
import time

m = ConcreteModel()
m.A = RangeSet(1000000)
m.p = Param(m.A, initialize=0, mutable=True)
m.v = Var(m.A, initialize=1)

i = sum(m.p[i] for i in m.A)

start = time.clock()
try:
    print(m.v[i])
except:
    pass
print(time.clock()-start)

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