`sum` method for AbstractArray{BigFloat}
`sum` method for AbstractArray{BigInt}
More efficient +,*,$,|,& methods for BigTypes

If the above functions are called with up to five arguments BigType
, the only allocation that will happen is of the return variable,
without any other temp variables.

This happens with +,* for BigFloats, and with +,*,$,|,& for BigInts.
Merge pull request #3065 from andrioni/master

Add `sum` methods for BigInts and BigFloats