Kotlin 2.0.0, performance improvements, bugfixes

• Fix for #280, return byte array containing zero instead of empty array when big integer is zero.
• Fix for #291, override hashCode as well as equals in ModularBigInteger.
• Fix for #292, negate for Modular big intiger did nothing.
• Bump to kotlin 2.0.0
• Add github action for pull request build and test.
• Many performance improvements in #299, #300, #301 #302, and many thanks to @glureau for implementing them!
• Fix for #306, mod inverse was invalid.
• Targets and sourcesets cleanup , thanks to @caffeine-mgn