Rework copy ctors

[marcauberer]

• Always generate a copy ctor if the struct contains a field of type struct that in turn has a copy ctor and/or if the struct has a heap field
• Make default copy ctor capable of copying the underlying buffer of owning heap-allocated fields
• Require a copy ctor for structs that are not trivially copyable
• On assignment, first check if a copy ctor exists, if not check if it is trivially copyable. If yes, do a shallow memcopy, if no throw a compile error
• Fix debug info on scope cleanup instructions
• Code quality improvements