FAST: Decimal support introduced

Individual operators for exponent and mantissa have not
been supported so far. This is to be implemented once
such necessity arises in practice.

Signed-off-by: Marat Stanichenko <mstanichenko@gmail.com>
[ penberg@kernel.org: kill 'decimal' typedef ]
Signed-off-by: Pekka Enberg <penberg@kernel.org>