Replace boost::container::flat_map with robin_map

[nvmkuruc]

Description of Change(s)

• Change _packValueFunctions to be a robin_map as it does not rely on the order provided by boost::container::flat_map
• Use robin_map to store allValuesAtAllTimes and create a sorted view of the map iterators for the subsequent order dependent operation

Fixes Issue(s)

• pxr/usd/usd/crateFile.{h,cpp} can use robin_map instead of boost::container::flat_map #2296

• 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-8038