Replace Identifier with String x Metadata

[joscoh]

Issue #, if available:

Description of changes:

Redefines an Identifier as String x IDMeta and parameterizes LExpr, LExprT, etc with IDMeta rather than Identifier. This allows several important operations on identifiers and expressions including:

• Generating arbitrary distinct identifiers (assuming IDMeta is Inhabited)
• Pattern matching on the name of a .op expression

Note: since the ToFormat instance of BoogieIdent (the only non-String identifier in use) simply gave the name, this implementation ignores IDMeta when converting Identifiers to String or Format.

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