Streamline parameter linkage

[dexter2206]

This PR introduces three essential changes:

• Changes data type of Routine.linked_params. Instead of mapping source parameters to pairs (routine, param), where routine is itself a Routine object, it now stores values of the form (path, param). This is done to facilitate detection of linkage deeper than one level without referring to routine's parent.
• Adds a precompilation stage that converts deep linkages into a series of one-level shallow linkages. This makes the compilation process (more) routine-local, which should move us closer to removing parent backlinking (Get rid of parent backlinking #28 ).
• Prohibits linkage of more than one level to be processed during the compilation stage, thus forcing user to either create a correctly defined routine with shallow linkages, or use an apropriate precompilation step (which is included in the defaults).

To explain what we mean by converting deep linkagkes into a series of shallow ones: suppose you have routine a, its child b and b's child c. Let a have parameter x linked to c's parameter y (i.e. a#x -> c#y). Then:

• The original link (a#x -> c#y) is removed.
• a#x is linked to a new input parameter b#c.y.
• b#c.y is linked to c#y.

Naturally, the same process applies if there is more than one level of nesting.