[TM-197] Change parser to make braces optional for embedded single instruction sequences

Description

This changes the contract parser so that the embedded sequences (ones that are part of an instruction) no longer require enclosing braces if they contain only a single instruction. The inline sequences, (including the top level one) still require them.

Related issue(s)

https://issues.serokell.io/issue/TM-197

Fixes by making enclosing braces for embedded instructions optional.

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