Fix calldata offset issue in PublicInput contract

[pinkiebell]

• fixes offset issue in contract
• improves the test_pi_commitment test

fixes: #128

[coveralls]

Pull Request Test Coverage Report for Build 4313150365

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.01%) to 90.138%

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Totals
Change from base Build 4302397224:	-0.01%
Covered Lines:	786
Relevant Lines:	872

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💛 - Coveralls