Introducing SDL struct

[TzuChieh]

Purposes:

• Solve the problem when input parameters have the same type and name: they can reside in unique structs now
• Reuse input parameter declarations and parsing, which is also cleaner
• Potentially benefit future implementation of OutputPacket and serializations

TODOs:

• Registering SDL structs
• A new SDL syntax for structs & parsing them
• Access SDL values and structs in a unified way (probably via TMP, and access like SDL references)
• Make sure encapsulating a struct with another one is possible