Add CachedExpressionConvertible

[braised-babbage]

This adds a new protocol, CachedExpressionConvertible, which can be used analogously to ExpressionConvertible but with support for the constructed expression to be reused during program construction. This can improve performance during construction of programs in which an underlying ExpressionConvertible object be used many times, or when the actual work of AST construction is non-negligible.

[PhilReinhold]

Aside from a docstring request, LGTM. Handing over to @jcjaskula-aws for braket perspective.

[braised-babbage]

I am sneaking in one unrelated change in this PR. I see that previously this was updated (cf 862d0c1), but OQIndexExpression was not actually moved. It should be exported as before.

[jcjaskula-aws]

I don't see any reason right now why this should be a problem for Braket. Would there be any drawback to do this with OQPyExpression instead of creating a new protocol?

[braised-babbage]

I don't see any reason right now why this should be a problem for Braket. Would there be any drawback to do this with OQPyExpression instead of creating a new protocol?

I had a vague concern about backwards compatibility here. One could very well imagine somebody constructing a program in which they mutate an ExpressionConvertible object during construction (e.g. in a Python loop over a set of parameters) and rely on Oqpy's current behavior (constructing fresh AST for each usage). I also think that not being able to opt out of this caching behavior could be annoying in certain circumstances.