[llvm-libtool-darwin] Refactor Slice and writeUniversalBinary

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