Rewrite standard equivalence library for full connectivity

This rewrites the standard equivalence library around the core of making
it fully connected, such that there are always paths through the
equivalence relations to translate a standard-library 2q gate into the
minimal number of standard-library 2q gates from a different local
equivalence class.  Similarly at the 1q level, the Pauli relations,
Clifford relations, and Clifford+T relations are arranged such that it
is always possible that a circuit in any of those classes can be
translated to any basis within the same class or higher.

Unfortunately, the current algorithm of the `BasisTranslator` is not
really suitable for such a library; its algorithm is designed and
efficient at finding _any_ translation, and cannot be directly weighted
to produce the _best_ translation when multiple possibilities are
available.  This new library in this commit requires a different
translation algorithm to realise its benefits. I call the new algorithm
the `BasisConstructor`, but it still needs writing into a transpiler
pass.

qiskit/circuit/equivalence_library.py

@@ -12,7 +12,9 @@
 
 """Session gates."""
 
-from .library.standard_gates.equivalence_library import StandardEquivalenceLibrary
+from .library.standard_gates.equivalence_library import (
+    STANDARD_EQUIVALENCE_LIBRARY as StandardEquivalenceLibrary,
+)
 from .equivalence import EquivalenceLibrary
 
 SessionEquivalenceLibrary = EquivalenceLibrary(base=StandardEquivalenceLibrary)