Add the directive form of a repetition (`!repeat`)

Add the directive form of a repetition (`!repeat`)

This patch introduces the first directive amongst other ones I'd like to
add later.

`!repeat` is equivalent to what's now called the post-item repetition
form:

    aa (bb cc dd) * 3 ee ff

    aa
    !repeat 3
      bb cc dd
    !end
    ff

`!repeat` (or the shorter `!r` alias) accepts exactly what's accepted
after a `*`: a constant integer, an expression, and (new) a name.

Obviously, the `!` symbol isn't considered a comment anymore.

See `README.adoc` and the new tests to learn more.

This patch also:

* Makes it legal to refer to the special `ICITTE` name within a
  repetition count expression.

  It's less natural for the post-item form, but it works nevertheless.

* Decouples the variables and labels dictionary types as `VariablesT`
  and `LabelsT`. Indeed, a variable value is (for the moment) an `int`
  or `float` instance, while a label value is always an integer (because
  it's an offset snapshot).

Change-Id: I77cd4c9f29639082612534940c48964a61e51392
Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>