Refactoring `Slice` class and function `createUniversalBinary` from
`llvm-lipo` into MachOUniversalWriter. This refactoring is necessary so
as to use the refactored code for creating universal binaries under
llvm-libtool-darwin.
Reviewed by alexshap, smeenai
Differential Revision: https://reviews.llvm.org/D84662