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

[nvmkuruc]

Description of Issue

CrateFile uses boost::flat_map to record a registry of type specific functions and a time code ordered list of values.

The first usage does not rely on the order of boost::flat_map and be replaced directly with robin_map.

The second usage requires an addition sort of the keys / iterators after the map has been populated.

Steps to Reproduce

N/A

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-8037

[nvmkuruc]

This was addressed by #2297 and can be closed.