BigInteger: refactor random number generation code somewhat
BigInteger: refactor random number generation code somewhat

terrafrost/phpseclib,paragonie-scott/phpseclib,danog/phpseclib,kenichiii/phpseclib,creatuity/phpseclib,thaigialai1987/phpseclib,thaigialai1987/phpseclib,melya/phpseclib,metaclassing/phpseclib,splitice/phpseclib,kenichiii/phpseclib,creatuity/phpseclib,paragonie-scott/phpseclib,splitice/phpseclib,danog/phpseclib,phpseclib/phpseclib,metaclassing/phpseclib,melya/phpseclib