Replace boost::iterator_facade with explicit iterator definition for pcp/iterator.h

[nvmkuruc]

Description of Change(s)

• Replace iterator_facade with explicit implementation for PcpNodeIterator, PcpPrimIterator, and PcpPropertyIterator.
• Provide _PtrProxy type for PcpNodeIterator and PcpPrimIterator (whose reference type is a proxy value and not a true reference) to provide a safe operator-> implementation.
• Use nullptr instead of 0 to default initialize pointer types (https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Res-nullptr)
• Use in-class initialization for defaults (https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rc-in-class-initializer)
• While testing removal of boost::reverse_iterator in some parallel work (Provide adapter for std::reverse_iterator to replace boost::reverse_iterator for proxy reference iterators #2333), it was observed that the current implementation of PcpNodeIterator hangs when std::prev was used. The source of the hang was not determined but not observed in this implementation. testPcpIterator has been augmented to include tests to validate that the increment / decrement operators and std::prev / std::next can be used and produce symmetrical end states.

Fixes Issue(s)

• Replace boost::iterator_facade usage with fully specified iterator classes #2305

• I have verified that all unit tests pass with the proposed changes

• I have submitted a signed Contributor License Agreement

[sunyab]

Filed as internal issue #USD-8089