ocamllex: user-definable refill actions (patch by Frédéric Bour)

The patch introduces a new "refill" action. It's optional and if
unused the lexer specification and behavior are unchanged.

When specified, it allows the user to control the way the lexer is
refilled. For example, an appropriate refill handler could perform the
blocking operations of refilling under a concurrency monad such as Lwt
or Async, to work better in a cooperative concurrency setting.

To make use of this feature, add

  refill {refill_function}

between the header and the first rule.

[refill_function] is a function which will be invoked by the lexer
immediately before refilling the buffer. The function will receive as
arguments the continuation to invoke to resume the lexing, and the
current lexing buffer.

More precisely, it's a function of type:

  (Lexing.lexbuf -> 'a) -> Lexing.lexbuf -> 'a

where the first argument is the continuation which captures the
processing ocamllex would usually perform (refilling the buffer, then
calling the lexing function again), and the result type ['a] should
unify with the result types of all rules.

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14461 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02