Implement CVariant functions

[zonical]

Very work in progress - working on it in small bits at a time

This PR aims to implement functions to CVariant which accesses data from variant_t on the C++ side. Useful for functions that hook into the Entity IO system. Unsure if a Set function is needed - but it can probably be done.

[zonical]

I've also added a function to get a string from a CUtlSymbolLarge class for plugin authors that wish to hook into AcceptInput without dealing with memory shenanigans on the C# side. Does not appear to have an easy way to modify it (from my little knowledge of hl2sdk) so it will remain as read-only.

[Yarukon]

you can just use fieldtype_t to replace VariantFieldTypes, fieldtype_t is generated from the schema