[#805] Better tests for `PACK`

This did uncover a couple bugs, but not exactly where we expected 😅

It might be a good idea to add property tests, too. It's at least feasible.

Description

Problem: It's important to make sure our implementation of PACK matches
that of tezos-client

Solution: convert all well-typed contracts into lambdas, pack them, and
compare that against reference.

Problem: Current implementation of CREATE_CONTRACT typechecking doesn't
switch mode when typechecking the contract itself. As a result,
typechecking lambdas that call CREATE_CONTRACT may fail with "SELF not
allowed in lambdas", despite the SELF instruction being inside the
contract.

Solution: set typechecking mode appropriately.

Problem: currently, implementation of CREATE_CONTRACT always uses
readable representation. This isn't quite right.

Solution: pass the selected representation along to `convertContract'`

Problem: we'd like to test our PACK implementation against reference.

Solution: add pack_data endpoint support to morley-client

Related issue(s)

Resolves #805 (closed)

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