BigFloats are indeed immutable "in spirit" but not really: julia> BigFloat.mutable == isimmutable(big(0.1)) true
So copy is a no-op (returns its argument as for every number), and deepopy
returns a new instance ("deepcopy(a) === a" is false), but then there is no
public API to mutate a or its copy. So there is no use for deepcopy on
BigFloats in client code, they behave as Float64.
