[perl #39329] Check to make sure PMC_str_val, etc. are used appropriately

[Matt Diephouse]

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PMC_str_val and family aren't guaranteed to work for subclasses, so
they should be limited to set_string, get_string, etc. String.pmc was
causing some failures for Tcl recently because of this. All the PMCs
should be checked for this sort of behavior.

--
Matt Diephouse