Change value type of TfUtf8CodePointIterator to TfUtf8CodePoint

[nvmkuruc]

Description of Change(s)

TfUtf8CodePointIterator was originally written before the TfUtf8CodePoint class was introduced. This change makes the value and reference type of the iterator a TfUtf8CodePoint directly instead of expecting clients to construct it themselves.

It also introduces a TfUtf8CodePointFromAscii helper function to make it easier to create code points from known ASCII characters.

This change also simplifies several test cases and usages. For example, _IsValidIdentifier no longer has to branch.

Currently, both the iterator and the code point class employ validation to ensure that code points outside the unicode range or surrogate code points are not decoded. It may be desirable to avoid this redundancy so that the validation is delegated entirely to the code point class in a future change.

Fixes Issue(s)

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

• I have submitted a signed Contributor License Agreement

[jesschimein]

Filed as internal issue #USD-9180