Allow renaming of generic types

For example if one wanted to change MyType_u32 to MyType_U32

I would like to be able to specify it in the export rename section:
```toml
"u32" = "U32"
```

This became apparent after testing #502 where we removed the `_` from name mangling, so we got MyTypeu32, and we want MyTypeU32, or even MyTypeUInt32