Move support for derived constructs to dedicated theories

[jeltsch]

Currently, support for certain derived constructs is part of the Þ-calculus proper. However, this blurs the distinction between fundamental bits, which actually constitute the Þ-calculus, and merely derived bits, which are more like DSLs.

We shall keep the derived constructs that are currently part of the Thorn_Calculus session in this session, as they are rather basic and moving them to some new session would contribte to proliferation of sessions. However, we shall move them to individual theories in a dedicated namespace Thorn_Calculus-Derived-Constructs. Later, we shall use the more general namespace Thorn_Calculus-Derived also for other things, in particular derived channel types.

The concrete construct that are affected by this issue are the following ones:

• repeated_receive
• guard
• general_parallel