Allow syntactic variable to be associated with values.

[lionelvillard]

Use case:

cterm :  VARIABLE<variable> csortanno?
             ...

Currently, because of the sort annotation, this grammar production corresponds to this enumeration sort:

data Cterm( Cterm_A1(String, List) ... )

which is an issue because the variable is not anymore a Cterm.

One possible solution is to allow syntactic variable to carry values, just like construction. In a sense, a syntactic variable is a special construction linked to a variable to allow substitution, which can be expressed as follows:

data Cterm ( variable Cterm_A1(String, List<Csortanno>) ... )

Notice the special variable keyword, replacing allows-variable.

variable by itself is still allowed and represent simple variable (with no additional values).