Split Expr from NavigableExpr with interpreter support

[TristonianJones]

Introduce a Go-native Expr representation

This PR splits the Expr interface from the NavigableExpr interface and introduces
support for the native value within the interpreter call paths.

The following changes will be occur in the following sequence once this PR is submitted:

• Update the type-checker to use the Expr value.
• Update the parser / unparser / pruner to use the Expr value.
• Introduce new Macro-related helpers to assist with backwards compatibility support
  for existing custom macros.

#789