RFR: JDK-8200436 - String::isBlank

[Jim Laskey]

New string instance method that returns true if the string is empty or contains only white space, where white space is defined as any codepoint returns true when passed to Character::isWhitespace.

webrev: http://cr.openjdk.java.net/~jlaskey/8200436/webrev/index.html
jbs: https://bugs.openjdk.java.net/browse/JDK-8200436
csr: https://bugs.openjdk.java.net/browse/JDK-8200437