NLv2 presolver: resolve constant defined vars/external functions

[jsiirola]

Fixes #3272 .

Summary/Motivation:

This resolves a number of errors and bugn in the NLv2 linear presolver:

• The ASL does not handle defined variables and external functions with constant subexpressions. This PR will simplify nonlinear subexpressions (within defined variables or arguments to external functions) to a single number (if fixing variables results in the subexpression becoming constant).
• improve detection of trivially infeasible models

Changes proposed in this PR:

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