Move the floating point code to cborinternal_p.h

... and put it behind #ifndef CBOR_NO_FLOATING_POINT.

The functions were not really compiler support, but are internal
functionality of TinyCBOR. This commit also puts both the <math.H> and
<float.h> headers behind an #if, so constrained systems without floating
point support won't enable it.

Signed-off-by: Thiago Macieira <thiago.macieira@intel.com>