Wrap Singular methods with thread safety checks.

[rbehrends]

These changes add a submodule Sync to Singular so that methods of the form Singular.f(...) can be called through Singular.Sync.f(...) in a thread-safe fashion.

[fieker]

what is the status here? Not complete? WIP? overlooked? Controversial? GoodToGo?
...
apart from the conflicts that is

[rbehrends]

I haven't updated it in a while, I will probably be able to make it current on Monday.

It currently runs on an opt-in basis, as there is non-trivial overhead involved in calling Singular through the thread-safe API. It allows you to use a thread-safe API (with identical method signatures and behavior) if you're running multiple Julia threads that access the C/C++ functionality of Singular concurrently. In theory.

Thus, it should not get in the way of normal operations, once the conflicts have been cleaned up.