Complete POPLMark Challenges

[wilbowma]

Based on reviewer feedback, Cur is still not convincingly realistic. I propose we complete the POPLMark challenges in Cur as a way to exercise the system, fix all issues we find along the way, and add abstractions /the Cur way/ to get to the right solutions (i.e., extend either of the two dimensions of Cur).

This is currently semi-blocked on the Turnstile core (branch, "turstile-core") to enable an extensible core logic,

• POPLMark:
  https://www.seas.upenn.edu/~plclub/poplmark/
• POPLMark Reloaded:
  https://lfmtp.org/workshops/2017/inc/papers/paper_8_abel.pdf
  https://homes.di.unimi.it/~momigliano/papers/poplr.pdf

[wilbowma]

(cc @stchang)

[wilbowma]

Work is begun in the "poplmark" branch. This currently requires a new branch of turnstile (https://github.com/wilbowma/macrotypes/tree/cur)