Fallback of floattype() for non-concrete types #342

On FPN v0.9.1, floattype() may return a type which does not belong to AbstractFloat for non-concrete types.

julia> floattype(Normed{UInt8, 8})
Float32

julia> floattype(Normed{UInt8})
Float32

julia> floattype(Normed{UInt64})
BigFloat

julia> floattype(Normed)
Normed

julia> floattype(FixedPoint)
FixedPoint

I don't think this will cause any practical problems for now, but considering promote_rule, I think it should return a BigFloat.
This might be a breaking change, but I think it can be treated as a bug fix.

Also, for information on disabling the "final" fallback for floattype, please refer to PR #177.