We want to get types into the IDA output, and to do that we need
accurate types for the Il2Cpp structures. Unfortunately, some crucial
types like Il2CppClass change between versions without any corresponding
metadata changes, meaning that we have to manually identify the version
outside of the Inspector somehow (e.g. by looking at the version number
embedded in Unity asset files). This patch adds header files for *every*
known Unity version from 5.3.0 to 2019.3.8, merging them into version
ranges where header files don't change.
It also adds front-end support for supplying the version number in both
the CLI and GUI. The GUI is given the ability to guess the version
number approximately to reduce the number of choices presented to the
user.
