Add BC-CNF pseudo boolean constraint encoder

[msakai]

The implementation is based on the paper:

• 南 雄之 (Yushi Minami), 宋 剛秀 (Takehide Soh), 番原 睦則 (Mutsunori Banbara), 田村 直之 (Naoyuki Tamura). ブール基数制約を経由した擬似ブール制約のSAT符号化手法 (A SAT Encoding of Pseudo-Boolean Constraints via Boolean Cardinality Constraints). Computer Software, 2018, volume 35, issue 3, pages 65-78, https://doi.org/10.11309/jssst.35.3_65

This is an incomplete implementation because our sequential encoder does not account for polarities.

There is room for improvement regarding the relationship between PB encoder, cardinality constraint encoder, and the Tseitin encoder.