[#544] Add `CAR k` and `CDR k` macros

Description

This MR adds the newest CAR k and CDR k macros introduced with edo. CAR k gets the 2k+1-th element of a right-combed pair, and CDR k gets the 2k-th element of a right-combed pair. Support for the new macros is also added in Lorentz.

For example, given the following right-combed pair:

    0      Depth: 0
   / \            |
  1   2           1
     / \          |
    3   4         2

The following observations are made:

• CAR 0 and CAR 1 are equivalent to CAR and CDAR, respectively.
• More specifically, CAR k is equivalent to CDR repeated k times followed by CAR.
• CDR 0 is equivalent to a no-op. CDR 1 and CDR 2 are equivalent to CDR and CDDR, respectively.
• GET 0 works with any type (not only pair) and so CDR 0 also works with any type.
• More specifically, CDR k is equivalent to CDR repeated k times.
• CAR k is simply { GET (2k+1) } and CDR k is simply { GET (2k) }.
• CDR k can be also thought as traversing the right-combed pair to a depth of k from the top.

Related issue(s)

Resolves #544 (closed)

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