Shave the yacc

Menhir vs Yacc
==============

Menhir being superior to ocamlyacc in almost every point, it may be worthwhile at some point in the future to consider using it as the parser generator for building Ocaml.

Of course this will come with a bootstrap problem, which I have no idea how to address and as such I am not proposing to switch the parser now.

However a second problem comes from differences in the grammar syntax and in the relevant APIs.

Menhir introduces the ```$startpos```, ```$endpos```, ```$startpos($id|ident)```, etc. keywords to refer to position of grammatical items in the production being reduced.

In contrast, ocamlyacc relies on the user manually querying the ```Parsing``` module to fetch the positions. This comes with shared global state incompatible with menhir approach.

A first step
============

The current pull request extends ocamlyacc to support a few features of menhir to help a potential migration, or even just to allow sharing the grammar between users of the two parser generators.

## Sugar

### Explicit names bound to RHS values

```
expr:
| LPAREN e = expr RPAREN { e }
```

Is now accepted and the ```e``` name is bound to ```$2``` in the action.

### Remove "=" as valid character to enter an action

```
expr:
| LPAREN expr RPAREN = $2 }
```

Was valid ocamlyacc code. Who would want to use that?! And of course, this is incompatible with the previous feature.

### Allow ocaml-style comments in the grammar

Until now, Ocamlyacc only supported C-style ```/* ... */``` comments in the grammar.  (Actions can of course embed ocaml-style comments).

Nested ```(* ... (* ... *) ... *)``` comments are now supported, with a limited support for strings inside comments (escaped character are just skipped, "tagged"-string literals introduced in Ocaml 4.02 lexer are not supported).

### Enable %start <type> TERM

The following code:
```
%start main
%type <Ast.t> main
```

Can now be written:
```
%start <Ast.t> main
```

## Bridging the gap

Most of menhir keywords can be used. 

```$startpos```, ```$endpos```, ```$startpos($id|ident)```, ```$startofs($id|ident)```, ```$endpos($id|ident)```,```$endofs($id|ident)``` are bound to the equivalent  call to ```Parsing.<…>```.

```$syntaxerror``` is equivalent to ```raise Parsing.Parse_error```.

```$previouserror``` fails at compile-time, because there is no way, AFAIK, to emulate this feature.

P.-S.
=====

In case of failure, these features try as much as possible to print a relevant error or warning message to the user.

And finally, having a grammar at the intersection of this ocamlyacc and menhir will also greatly help merlin in supporting new versions of the grammar :).