Add a path datastructure to constraints

[jaapdejong15]

There should also be a path datastructure in the constraints, accompanying the tree datastructure.
It should define a constraint on the path from the root to the current hole.

Furthermore, it should also support variables, e.g. 1-x-2, meaning that there is a variable number of nodes allowed between rulenode 1 and 2. This can be used to e.g. define the ComesAfter constraint.

Nice-to-haves:

• Define a syntax with which users can easily provide these paths
• Support for defining options, e.g. [1,2] matches either rulenode 1 or 2
• Support for various regex-like symbols, such as
  • + for matching 1 or more
  • ? for matching 0 or 1
  • * for matching 0 or more
  • . for matching every rule
  • r{n} for matching rule r exactly n times. This could also be desugared.