Create CallingConventionSig base class and add more MethodSig ctors
Create CallingConventionSig base class and add more MethodSig ctors

yck1509/dnlib,ilkerhalil/dnlib,modulexcite/dnlib,ZixiangBoy/dnlib,Arthur2e5/dnlib,kiootic/dnlib,jorik041/dnlib,0xd4d/dnlib,picrap/dnlib