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Use singletons for standard library unparameterized, non-controlled gates (#10314)

* Use singletons for standard library unparameterized, non-controlled gates

This commit adds a new class SingletonGate which is a Gate subclass that
reuses a single instance by default for all instances of a particular
class. This greatly reduces the memory overhead and significant improves
the construction speed for making multiple instances of the same gate.
The tradeoff is in the flexibility of use because it precludes having
any potentially mutable state in the shared instance. This is a large
change to the data model of qiskit because it previously could be
assumed that any gate instance was unique and there weren't any
unintended side effects from modifying it in isolation (for example
doing XGate().label = 'foo' wouldn't potentially break other code).
To limit the impact around this instances of SingletonGate do not allow
mutation of an existing instance. This can (and likely will) cause
unexpected issues as usage of the class is released. Specifically what
used to be valid will now raise an exception because it is a shared
instance. This is evident from the code modifications necessary to
most of the Qiskit code base to enable working with instances of
SingletonGates. The knock on effects of this downstream are likely
significant and managing how we roll this feature out is going to be
equally if not more important than the feature itself. This is why
I'm not personally convinced we want to do all this commit includes
in a single release. I've opened this as a pull request primarily to
start the conversation on how we want to do the roll out to try and
minimize and notify downstream users of the potential breakage to
avoid issues. The primary issue I have is this doesn't really follow
the Qiskit deprecation policy as there is no user facing notification
or documentation of this pending change and code that worked in the
previously release will not work in the ... (continued)

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