Replace boost::iterator_facade with explicit implementation for SdfChildrenView's internal iterator

[nvmkuruc]

Description of Change(s)

• Replace iterator_facade with explicit operators for SdfChildrenView::_InnerIterator
• Use nullptr instead of NULL 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)
• Use using instead of typedef for defining iterator traits (https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rt-using)

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-8253

[crevilla]

I'm pretty sure you're going to need operator-> here too. This iterator is the type for SdfChildrenView::const_iterator (after all the template expansion) so it's highly likely that we have Presto code that dereferences via ->