Case-sensitive changes to ARCH enum

[GregIthaca]

This is more of a suggestion really: it might be good to make a note in the change log about this change.
In lief <= 0.14 the lief.ELF.ARCH class contained i386 and x86_64; in lief >= 0.15 it now has I386 and X86_64.
While this is more consistent, it's a breaking change for older code that has the values in lower case, and I couldn't find it mentioned anywhere in the documentation.

[romainthomas]

Hi @GregIthaca
You are right. I'm sorry if you faced issues to migrate from LIEF 0.14.x to 0.15.x because of this change. I try to keep the changelog updated and accurate but I'm prone to errors.