Approximate element pairing for sets

[spavikevik]

This tries to implement #66 in the following manner:

• Introduces difference counts (one for the number of differences, and another for the number of ignored differences)
• Abstracts pairing functions into PairingFn which can be equals-based or differ-based:
  • Equals-based functions work the same way as they used to
  • Differ-based pairing can be either approximate, using the default item differ or can rely on a user-specified differ instance. This is to allow for user customizability.
  • Differ-based pairing functions take an additional parameter for difference count threshold.
• The difference count is compared against the threshold specified in the pairing function to then match similar elements.

I tried to approach this in as common-sense way as possible and made some assumptions when implementing this such as using the approximate pairing function by default and specifying difference threshold to be 1, but I am open to discussing/adjusting them as needed.