Use `Il2CppSystem.String` instead of `System.String`

[ds5678]

This also includes the removal of Il2CppStringArray and calling a conversion operator after ldstr during unstripping.

• There are implicit conversion operators defined, so this shouldn't create extra friction in code.
• String marshalling is expensive, and in many cases, eliminating it might lead to performance improvements.
• Quite often the string is only read/tested, which is the primary scenario where performance would be gained.