boost::compressed_pair can be replaced with explicit empty base optimization

[nvmkuruc]

Description of Issue

boost::compressed_pair is being leveraged in TfDense{Map,Set} to take advantage of the empty base optimization for the Hashing and Equality types. This can be done explicitly with private inheritance on the containers.

Longer term, the [[no_unique_address]] attribute should be considered when C++20 is an option.

Steps to Reproduce

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System Information (OS, Hardware)

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Package Versions

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Build Flags

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[nvmkuruc]

I have a fix.

[tallytalwar]

Filed as internal issue #USD-7991

[nvmkuruc]

This was addressed by #2522 and #2257