[#322] Fix `CONTRACT` interpretation with ambiguous entrypoints

Description

Problem: The CONTRACT instruction is specified to result in None when both the instruction and the input address provide a non-default entrypoint. morley was incorrectly throwing a MichelsonAmbigousEpRef exception in this case.

Solution:

• The interpreter is fixed to produce None.
• The MichelsonAmbigousEpRef exception is removed.

Related issue(s)

Resolves #322 (closed).

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