Multiple dispatch for relational expression generation

[jsiirola]

Fixes # .

Summary/Motivation:

This is a continuation of the work moving Pyomo expression generation to use multiple dispatch for more efficient / standardized argument processing. This migrates the relational expression generation. Overall, it provides more standardized and efficient argument processing. Overall performance on the performance test suite is improved (in particular, a 3.5% improvement in create_instance:

Changes proposed in this PR:

• Move relational expression generation to use multiple dispatch
• Move some definitions (enums and dispatcher utilities) into expr_common
• Improved solution to managing the circular dependency between relations and numeric expressions (with better code documentation)

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[jsiirola]

(Do not attempt to squeeze this into the release)