This commit adds (**) as a special empty documentation comment that
can influence the attachment of other documentation comments but is
never emitted as an attribute in the parsetree. An important use of
this empty comment would be to attach a documentation comment to the
type t and not its last constructor Label, without spurious empty
attribute, in the following construction:

type t = Label (**)
(**doc for t*)

Remains to be done: remove all headers in testsuite/tests.

PConst -> Pconst
int -> integer

.. and allow any letter in [g-zG-Z] as modifier (previously 'l','L','n')
Also allow modifier for floats
This give more freedom to ppx rewritters (what about a ppx for zarith)

Checks are performed when translating from Parsetree to Typedtree.
Invalid_literal is raised if the modifier is not recognized ([lLn]?)
Integer_overflow is raised as before.

Lexer: use g-zG-Z for integer literal modifier

Lexer: Allow modifier on float

Clean wrt previous commits

Lexer: use named substring

Cleanup

typo

doc

fix after rebase

rebase on  trunk

Update typecore.ml

Fix printast.ml

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@16415 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02