Synthetic name of serializable lambda methods should not contain negative numbers

Serializable lambdas are desugared to methods where name follows following pattern:

     lambda$mmm$kkkk$nnn

Where mmm is the method name and kkk is the hashcode of the method signature, and nnn is a sequentially assigned number. That way, dependencies on lambdas from other methods will be minimized.

If hashcode is a negative number the impl will add a '-' to the method name - we should aboid that, i.e. by using an hexadecimal representation of the hashcode.