[#720] Allow code entry take single command

Description

Problem:

• We assumed, that code entry must contain a sequence, but there also can be single instruction.
• We assumed, that DIP (and other commands, taking code as argument) can take single instruction, but it must take a sequence.

Solution:

• Make operations parser accept only sequence.
• Make code entry parser accept sequence or single instruction.

Related issue(s)

Resolves #720 (closed)

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