Improve server model

[lars-reimann]

Is your feature request related to a problem? Please describe

The server model has a few shortcomings.

Tasks

• Infer the name from the qualified name or vice versa.
• Store imports needed for typing and imports needed for calls separately. Only the type-related imports should appear in stubs.
• Represent calls (qualified name of called function + arguments); must also work for enums/parameter objects; then parameters no longer need a link to the original declaration
• Represent arguments (expression + optional name)
• Represent references (e.g. to parameter)
• Represent values/literals (string, int etc.). Useful for calls and default values.
• Represent constraints (e.g. boundary) -> create an if + raise in codegen; also rename boundary to constraints