Implement threading

[DrSloth]

A simple threading API with a spawn function returning a JoinHandle.
JoinHandles then can be waited on with the join function or are implicitly waited on if access to the data is required.

[DrSloth]

For more advanced purposes this depends on #18

[DrSloth]

Simple threading with spawn and join is ready now. The next step needs a global var manager API that is also needed for references and lazy.

[DrSloth]

#19 and #16 also have something todo with the changes