Support negative and binary values in log encoding (#248)

* partially support negative values in log encoding

* factor out encode_linear_log_from_encodings

* support negative values in encode_mul_log

* use log encoding in fuzz.rs

* fix

* force log encoding in fuzzer

* fix

* support binary vars in encode_var_log

* bug fix

* reduce rounds for fuzzer with log encoding

Author: semiexp