[numeral notation] R

Previously real constants were parsed by an unproved OCaml code. The
parser and printer are now implemented in Coq, which will enable a
proof and hopefully make it easier to maintain / make evolve.

Previously reals were all parsed as an integer, an integer multiplied
by a power of ten or an integer divided by a power of ten. This means
1.02 and 102e-2 were both parsed as 102 / 100 and could not be tell
apart when printing. So the printer had to choose between two
representations : without exponent or without decimal dot. The choice
was made heuristically toward a most compact representation.

Now, decimal dot is parsed as a rational and exponents are parsed as a
product or division by a power of ten. For instance, 1.02 is parsed as
Q2R (102 # 100) whereas 102e-2 is parsed as IZR 102 / IZR (Z.pow_pos
10 2).

1.02 and 102e-2 remain equal (proved by reflexivity) but 1.02e1 = Q2R
(102 # 100) * 10 and 10.2 = Q2R (102 # 10) no longer are.

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