Add subscript rvalues management in grammar and AST transformations

In C++ words, grammar and AST now deal with `operator[]` for rvalues (right hand
side values). The syntax now allows to write
``
a[i];
``
which defines a new expression `a[i]` and where both `a` and `i` are expressions.

It remains to do the semantic analysis to check if `a[i]` means something, and
then to get the associate value.

lvalues threatment, (that is to assign a value to `a[i]`) will be performed later.