Rule out most FIXMEs

* Fix precedense ordering of expr
* Code style: leaner writing of plural-rules
* fun_func requires '=' instead of lra
* Revert special lexing case 'for PHP programmers'
    : no incentive to use such /reserved/ atoms
* Forbid lexing - in atoms
    (- would replace _ also allows Erlang's syntax for binaries)
* Replace if by cond
* Add a one-line if. else = if not
    : no new keyword here
* Update/Add tests in consequence
* Still needs to fix precedence in map & = for fundef
Also reorder matchable expressions.
Support again = instead of -> in defs.