Improve bidict implementation and tests

[elliottslaughter]

• Implement bidict::equate, bidict::equate_strict, bidict::erase_l, and bidict::erase_r using only local checks
• The semantics of bidict::equate (non-strict) are that conflicting keys are ERASED in order to preserve bidict consistency, so use it only when that's what you want
  • For the record, all existing users would be satisfied with bidict::equate_strict, but I'm trying to keep this PR small so I didn't update them
• Run entire-bidict tests only at construction time
• Update unit tests to test more of the interface (particularly constructors, mutation) and in the order that methods are declared in the original class

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