Introspection / debugging helpers

• sgs_Stat - retrieve or dump various parts of VM state
• sgs_StackFrameInfo - retrieve preprocessed info from stack frame
• sgs_GetFramePtr - return a stack frame pointer
• sgs_DebugDumpVarExt - push a string containing variable data and return const char* to it
• sgs_DebugDumpVar - push a variable dump string and return const char* to it
• sgs_DebugPrintVar - push a string, converted from variable and return const char* to it
• sgs_CodeString - retrieve a name string for the specified constant
• sgs_WriteErrorInfo - write message info to output callback
• sgs_PushErrorInfo - push message info to stack