Implement a preprocessing phase from SCode to SCode that simplifies instantiation

[Adrian Pop]

Implement an preprocessing phase that removes:

• extends (copies the components from base class and applies the modifications)
• imports (transforms all type names to fully qualified ones)
• redeclare (applies the redeclares by replacing the redeclared parts)
• fully qualifies the class names (SCode.name becomes SCode.path).

This will benefit us several ways:

• instantiation will be easier and more robust.
• lookup will be less complex
• error messages will be better
• API functionality will not require partial instantiation

[Per Östlund]

This has mostly been implemented, and the parts not yet implemented are instead part of the plans for the new instantiation.