refactor(isthmus): strptime function mapping

[bestbeforetoday]

Avoid code duplication. Also test operator name and operands explicitly instead of relying on Calcite's toString implementation, which might change over time.

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[nielspardon]

Also test operator name and operands explicitly instead of relying on Calcite's toString implementation, which might change over time.

I recall that in the past when I added unit tests that there was a preference to rely on Calcite's explain() / toString() implementation for readability of the tests. I guess we should get some clarity on that. Otherwise I'm fine.

[bestbeforetoday]

I generally avoid using toString since the Javadoc for Object.toString has this API note (my emphasis):

In general, the toString method returns a string that "textually represents" this object. The result should be a concise but informative representation that is easy for a person to read. It is recommended that all subclasses override this method. The string output is not necessarily stable over time or across JVM invocations.

I don't see anything in the Calcite Javadoc that confirms the toString format or that it is stable across releases.

For now I have just reverted to using toString. It can be revisited later if it ever breaks.

[nielspardon]

if you want to have even more reuse then PositionFunctionMapper also just swaps the position of the first two arguments of its function call

[bestbeforetoday]

This refactor consolidates a family of closely related date/time parsing functions that require the same argument mapping and differ only by the type they return. POSITION deals with string searching so, while it also requires the same argument mapping, the relationship is not obvious and could be confusing. I would prefer to keep this PR focused on removing duplication in the closely related date/time parsing functions.