encoding number types should not use both Int and Float families

[mattbishop]

When msgpack-lite encodes a number primitive type, the actual encoded value will be either in the Int family or the Float family. While the msgpack spec does allow for serializing to the smallest-possible type in a given family, it doesn't allow for crossing over to another family. See https://github.com/msgpack/msgpack/blob/master/spec.md#serialization-type-to-format-conversion

Given that JS has no way to clarify int vs float values, I would like to see the addition of a Float class to msgpack so that a developer could explicitly state a number is in fact a float, event if the value itself can be interpreted as an int.

As a bonus, it would be nice to accept a Long value as well so that one does not have to manually pass in Int-64 objects.

The addition of these two class types would give developers control over the family used to encode their values. The existing number support can stay for backwards compatibility.