BigInteger: refactor randomPrime somewhat
BigInteger: refactor randomPrime somewhat

melya/phpseclib,paragonie-scott/phpseclib,phpseclib/phpseclib,metaclassing/phpseclib,phpseclib/phpseclib-php5,creatuity/phpseclib,kenichiii/phpseclib,danog/phpseclib,paragonie-scott/phpseclib,terrafrost/phpseclib,creatuity/phpseclib,thaigialai1987/phpseclib,phpseclib/phpseclib-php5,melya/phpseclib,kenichiii/phpseclib,splitice/phpseclib,splitice/phpseclib,danog/phpseclib,thaigialai1987/phpseclib,metaclassing/phpseclib