No more deprecation warnings.

Modified [PreFront.read_whole_file] to use [IO.exhaust].

This removes two eta-redexes.
The speed difference is barely measurable, maybe 0-5%.

when --table and --external-tokens were used.

Menhir even if it has not been installed.

IncrementalEngine, so as to reduce confusion and allow
including this file in the documentation.

and the incremental API.

even Menhir has not been installed yet.

This costs one more field in the [env] record.
The speed difference is actually not measureable.
Maybe it would be measureable in an application where the [env] records
  are stored and move to the major heap. Never mind.

the incremental entry points in the generated [.mli] file.

version of [start] for each starting state. This should give the user a
safe API for invoking the incremental parsing functions.