Merge: Fixints

As promised, a check off the TODO list for #1266 and for low-level users: fixed-size Integers.

A literal rule has been added for these types, namely:

* i8: Signed Integer 8 bits
* i16: Signed Integer 16 bits
* i32: Signed Integer 32 bits
* u16: Unsigned Integer 16 bits
* u32: Unsigned Integer 32 bits

Note that an Unsigned Integer 8 bits is missing because of Bytes, which gets to keep its `u8` suffix until we decide of something

Pull-Request: #1588
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>