Generate multiple patterns with circuit_validator

[lee30sonia]

This PR:

• Adds an interface generate_pattern to generate multiple different PI patterns for a node to be a certain value in circuit_validator. This will be used in another PR for pattern_generation. I only implement this method for validating with a constant value because I only use it here. But if generating multiple CEXs in other types of validation is needed in other applications, it is also possible to be implemented in the future.
• Fixes a bug when providing the constant node as the first argument of validate. (Actually just removing unnecessary assertions.)

[coveralls]

Pull Request Test Coverage Report for Build 154580216

• 27 of 29 (93.1%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 80.16%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
include/mockturtle/algorithms/circuit_validator.hpp	27	29	93.1%

Totals
Change from base Build 153854969:	0.03%
Covered Lines:	8921
Relevant Lines:	11129

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💛 - Coveralls

[hriener]

Thanks.