Factor SCM_IDENTIFIER out from embedded_scm_arg*

Factor SCM_IDENTIFIER out from embedded_scm_arg*

SCM_IDENTIFIER indicates "active Scheme" introduced with $ that is
subject to copying and reinterpretation.  While the parser had its own
*_IDENTIFIER category for everything interpreted specially, the
fallback SCM_IDENTIFIER itself never needed special or different
treatment from material generated with # (SCM_TOKEN).  As the various
*_IDENTIFIER types are faded out, this is changing.  As a consequence,
SCM_IDENTIFIER no longer shares the non-terminals with other
Scheme-type definitions.