* m4/m4module.h (M4_SYNTAX_ASSIGN): Delete this unused

* m4/m4module.h (M4_SYNTAX_ASSIGN): Delete this unused
extension to make enough room.
(M4_SYNTAX_IGNORE): Change value to zero, since the (commented)
code in input.c special cases this category.
(M4_SYNTAX_LBRACE, M4_SYNTAX_RBRACE): New values in preparation
for define(eleventh,${11}) extension.
* m4/syntax.c: Update comments accordingly.
(m4_syntax_create, m4_syntax_code): Handle { and }.
* m4/input.c (m4__next_token): Likewise.
* src/freeze.c (produce_frozen_state): Support new categories.
(reload_frozen_state): When loading version 1, disable syntax
features not present in 1.4.x.
* tests/freeze.at: Sort and rename existing tests.
(loading format 1): Ensure we don't break v1 frozen files that
used raw ${.
(loading format 2, reloading changecom, reloading changesyntax):
New tests.
* doc/m4.texinfo (Changesyntax, Frozen file format 1): Document
the new syntax categories.
(Arguments): Update the FIXME comment.
(M4wrap): Merge examples from branch, before worrying about fixing