"pure" and "error" special forms

[jmcardon]

cc @CryptoPascal31

Supersedes #273 (Saves me a rebase).

Implements KIP 0024.

This PR adds two special forms into pact.

pure

(pure k) for some expression k will evaluate k in read-only mode. That is, the evaluation of k cannot write to the database, only read.

Example here

typing rule

  Γ ⊢ e : t
───────────────  (pure)
  Γ ⊢ (pure e) : t

Other notes

pure has been a community ask for a while, and allows users to use the power of try but without forcing users to write `(try (enforce false "boom") something)

error

(error foo) for some string valued expression foo (e.g "hello" or (format "{}" ["bar"])) will throw a recoverable error (so catchable with try).

Example here

typing rule

For some fresh α

  Γ ⊢ e : string
──────────────────────  (Error)
  Γ ⊢ (error e) : α

error in pact is essentially ⊥-elimination for pact. This is particularly useful for typechecking expressions where (enforce false will not suffice, as the return type of that is bool.

PR checklist:

• Test coverage for the proposed changes
• PR description contains example output from repl interaction or a snippet from unit test output
• (If Relevant) Documentation has been (manually) updated at https://docs.kadena.io/pact

Additionally, please justify why you should or should not do the following:

• Benchmark regressions
• Confirm replay/back compat (Ignore until core release)
• (For Kadena engineers) Run integration-tests against a Chainweb built with this version of Pact (Ignore until core release)