Syntax tree nodes for language module expressions, of type
Ast.module_expr. The basic nodes are:
<:module_expr< $me1$ . $me2$ >>: access in module.
<:module_expr< $me1$ $me2$ >>: application.
<:module_expr< functor ( $s$ : $mt$ ) -> $me$ >>: functor.
<:module_expr< struct $list:stl$ end >>: structure.
<:module_expr< ( $me$ : $mt$ ) ] >>: module type constraint.
<:module_expr< $uid:i$ >>: identifier starting with an
uppercase letter.