added support for float & double

[ssyram]

As the title, support for float & double, namely, Float32 and Float64 are added to the Constant.ml. Relevant parts are fixed. This is required from the project Eurydice.

Question left: where to add the float32_t and float_64_t definitions? Just adding to Eurydice is fine, but if this is not defined, Krml might be less self-contained.