Use in class initializers to simplify token macros

[nvmkuruc]

Description of Change(s)

In class initializers reduce the number of internal macros required as well as the number of boost headers required to produce static tokens objects. This introduces a dependency onto boost/preprocessor/seq/enum.hpp to convert sequences into standard variadic arguments. This retains the dependency on boost/preprocessor/seq/for_each.hpp and boost/preprocessor/seq/filter.hpp to continue support for arrays.

While this change does not update the interface to support this, in class member initializers also allow for the public token macros to define tokens inline akin to the private macros.

/** header **/
#define MY_TOKENS \
   (abc)                       \
   (xyz)

TF_DECLARE_PUBLIC_TOKENS(MyTokens, MY_TOKENS);

/** source **/
TF_DEFINE_PUBLIC_TOKENS(MyTokens, MY_TOKENS);

could become

/** header **/
TF_DECLARE_PUBLIC_TOKENS(MyTokens,
   (abc)
   (xyz)
);

/** source **/
TF_DEFINE_PUBLIC_TOKENS(MyTokens);

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-8954