Introduce `ofTypeEqual` and use in generated constraints

[lmsurpre]

I'm wondering if we should also introduce ofTypeEqual and use that in our generated expressions instead of asTypeEqual.

The background on this one is pretty interesting: many of the fhirpath expressions in the base spec (and a few of the constraints) use the fhirpath as function to convert a list of input items to a filtered list of output items of a given type (or subtypes).

However, the official as documentation says this about it:

If there is more than one item in the input collection, the evaluator will throw an error.

Furthermore, the spec lists the as function as deprecated, although I think its more to favor the as operator instead (which is not retired).

Anyway, I think most of those expressions and FHIRPath constraints should probably be using the ofType function instead of as.

I think its too much of a breaking change to make our as function spec compliant at this point, however I do think that we could introduce ofTypeEqual and use that instead of asTypeEqual in our generated constraints.

Originally posted by @lmsurpre in https://github.com/LinuxForHealth/FHIR/issues/3974#issuecomment-1276971570

[lmsurpre]

Here is a JIRA issue I opened for the dicrepency between the FHIRPath as definition and its usage in FHIR: https://jira.hl7.org/browse/FHIR-25188

Although you can see that they are also starting to fix some of the expressions to favor ofType over as: https://github.com/HL7/fhir/pull/2395/files https://github.com/HL7/fhir/pull/2549/files