pkSetGlobal function implemented to support adding globals from the
native interface. Also the PKVM slots doesn't need a runtime anymore to
use them (see math.PI global to see how it works).
pkVar is removed and slots (like wren) are implemented for accessing or passing pocketlang values to C and vice versa. Yet all the slot related functions are callable at runtime (a fiber is associated with the VM) which needs to be refactored to allocate slots even if it's not runtime, to let users make use of the pocketlang values before or without running a function.