Reduce overhead of m4_builtin([defn]).

Reduce overhead of m4_builtin([defn]).

* lib/m4sugar/m4sugar.m4 (_m4_defn, _m4_popdef, _m4_undefine): New
internal macros, which are slightly more efficient than
m4_builtin([defn]) and company.
(m4_defn, m4_popdef, m4_undefine, m4_warn, m4_ifset)
(_m4_dumpdefs_up, _m4_dumpdefs_down, _m4_wrap, m4_for)
(_m4_divert_n_stack, m4_divert_pop, m4_expansion_stack_push)
(m4_expansion_stack_dump, _m4_defun_pro, _m4_defun_epi)
(_m4_defun_epi_outer, _m4_require_call, m4_combine, m4_append)
(_m4_append_uniq, m4_append_uniq_w, _m4_text_wrap, m4_text_box)
(m4_version_prereq): Use them.

Signed-off-by: Eric Blake <ebb9@byu.net>