Serialization/conversion performance improvements

[mattwthompson]

• Tag issue being addressed
• Add tests
• Update docstrings/documentation, if applicable
• Lint codebase
• Update changelog

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[mattwthompson]

Bummer, the behavior seems fine:

In [3]: hash(ForceField("openff-2.0.0.offxml")['vdW'])
Out[3]: -7474900899989015509

In [4]: hash(ForceField("openff-2.1.0.offxml")['vdW'])
Out[4]: -7474900899989015509

In [5]: hash(ForceField("openff-2.2.1.offxml")['vdW'])
Out[5]: -2512421923988562758

and the raw performance isn't terrible:

In [6]: %%timeit
   ...: hash(ForceField("openff-2.2.1.offxml")['vdW'])
   ...:
   ...:
80.5 ms ± 1.44 ms per loop (mean ± std. dev. of 7 runs, 10 loops each)

but something somewhere is causing huge slowdowns, or this just doesn't scale well to large systems. That's what's up next.

[mattwthompson]

I don't think there was a reason to convert this quantity into the units it already has

[mattwthompson]

This check is not free, but it improves the current design by not checking for indexed or indexed + mapped attributes if an attribute name is alphabetic