Fix an error in CodeGen that failed to get System.Type (Unity 2021)

[hadashiA]

#187
Unity 2021 seems to give an error when building with CodeGen. (You can run it in the editor).

The probrem:

• System.Type.GetType in CodeGen is broken. Probably the assembly to be compiled cannot be referenced at build time.

Fix:

• Try to use Assembly.Load when Assembly cannot be referenced

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