Variable type codes

• SGS_VT_NULL: null
• SGS_VT_BOOL: boolean
• SGS_VT_INT: integer
• SGS_VT_REAL: real value
• SGS_VT_STRING: string
• SGS_VT_FUNC: SGS function
• SGS_VT_CFUNC: C function
• SGS_VT_OBJECT: object
• SGS_VT_PTR: pointer
• SGS_VT_THREAD: thread/coroutine/context