applicative module paths in signatures

    type [params] id := type_expr { and [params] id := type_expr }
    module Uid := extended-module-path

- The code responsible for printing Syntaxerr errors is moved to the
  Parse module (so that it can depend on the variable printer in
  Pprintast).

- Pprintast becomes a dependency for a few tools that link some
  compiler modules in an ad hoc way (they would better be implemented
  in terms of compiler-libs).

Type variables whose name has a single quote as the second character
(such as ' a' or ' a'b) need to be printed with a whitespace after the
initial quote symbol, in order not to be confused with a character
literal.

The concrete syntax only allows attributes on tags/constructors/fields
(Rtag, Otag), not on inherited subtypes (Rinherit, Oinherit); we add
this as new enforced invariant in ast_invariants.

Florian Angeletti and myself ran into a problem when trying to use attributes
for ellision of parts of manual example. We wanted to be turn any ast-node
marked with the [@ellipsis] attribute into "..." in the rendering of the
corresponding code block, but for this we need the location of the
attributed node, and it turns out that some constructions supported
attributes without carrying a location:
- Rtag in row_field
- Otag in object_field
- type_exception record
- type_extension record

We added locations in all those positions, guaranteeing the invariant
that all nodes to which attributes can be attached have a precise
position.

Changes

fix typo