Denial of service when feeding malformed size arguments in go-bitfield

go-bitfield is a simple bitfield package for the go language aiming to be more performant that the standard library. When feeding untrusted user input into the size parameter of `NewBitfield` and `FromBytes` functions, an attacker can trigger `panic`s. This happen when the `size` is a not a multiple of `8` or is negative. There were already a note in the `NewBitfield` documentation, however known users of this package are subject to this issue. Users are advised to upgrade. Users unable to upgrade should ensure that `size` is a multiple of 8 before calling `NewBitfield` or `FromBytes`.