* module/mes/psyntax-1.mes (syntax-rules): New syntax transformer,
based on syntax-case.
(define-syntax-rule): New macro.
(let-syntax): New macro, for syntax-case.
* tests/let-syntax.test: Switch to syntax-case.
* tests/psyntax.test: Add syntax-rules and syntax-rule test.
* mes.c (sc_expand_env): New function.
(builtin_eval): Use it.
* module/mes/psyntax-0.mes: New file.
* module/mes/psyntax-1.mes: New file.
* tests/psyntax.test: New file.
* GNUmakefile (TESTS): Add it.