feat: void return gadgets, multiple circuits export and Vector optimisation

[Eagle941]

Summary

• Added support for gadgets which return void.
• Added support for extraction of multiple gadgets/circuits with single function call.
• Implemented frontend.Compiler interface for CodeExtractor and implemented functions Field() and FieldBitLen().
• Added optimisation for Vectors extraction: the extractor checks if it's possible to pass the whole vector instead of reconstructing it.
• Added support for gadgets returning n-dimension frontend.Variable slices
• Added support for gadgets returning nothing
• Added better error handling, catching a panic and returning an error

Plus other stability hotfixes.

Details

Stability hotfixes:

• Support for nested slices in gadget initialisation (achieved through function flattenSlice)
• Explicit coercion to F type for Const fields, except for int types (achieved by creation of a new type Integer)
• Added check in abstractor.CallGadget to make sure the argument is of type frontend.API
• Checking no whitespace is present in namespace tag
• nil input variables initialised to 0
• Fixed bug in function cloneGadget which wouldn't deep copy nested slices. I have imported the library mitchellh/copystructure as a better alternative to homebrewed solution.

Additional test examples included plus automatic check of test output with reference extracted circuit.

Checklist

• Documentation has been updated if necessary.