Replace usage of boost::compressed_pair with explicit empty base optimization

[nvmkuruc]

Description of Change(s)

• Update TfDenseHash{Set,Map} to explicitly use private inheritance to take advantage of the empty base optimization instead of indirectly through boost::compressed_pair
• Decouples initialization of vector storage, HashFn, and Equality{Key,Element} in constructors
• Updates constructors to use std::make_unique instead of new operator (https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rr-make_unique)
• Decouples swapping of vector storage, HashFn, and Equality{Key,Element} in swap methods
• Remove references to compressed_pair in test for TfDenseHashMap
• Remove spurious include of utility.hpp in pxr/base/tf/denseHash{Set,Map}.h
• Remove spurious include of boost/compressed_pair.hpp in pxr/usd/sdf/childrenView.h

Fixes Issue(s)

• boost::compressed_pair can be replaced with explicit empty base optimization #2257

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

• I have submitted a signed Contributor License Agreement

[tallytalwar]

Filed as internal issue #USD-7993

[crevilla]

Okay, we don't want to do this this way. The semantics of TfDenseHashMap now BEING a HashFn and an EqualKey is not desirable.
What I think you really want to do is reimplement the internal member class _VectorHashFnEqualFn using empty base optimization explicitly instead of the compressed_pair

[crevilla]

Same comment here; we don't want the DenseHashSet inheriting from these function types.