RFR 8251989: Hex encoder and decoder utility

[Roger Riggs]

Please review a java.util.Hex API to encode and decode hexadecimal 
strings to and from byte arrays.

Within the JDK and JDK tests there are multiple implementations to 
encode and decode
hexadecimal strings to byte arrays. Hex encoders and decoders support
upper or lower case hexadecimal characters, delimiters, prefix, and suffix.
The API is modeled after the java.util.Base64 API providing static 
factories,
immutable threadsafe instances with methods to encode to and decode from
string and StringBuilder.

JavaDoc:
http://cr.openjdk.java.net/~rriggs/hex-javadoc/java.base/java/util/Hex.html

Webrev for Hex encoder and decoder:
    http://cr.openjdk.java.net/~rriggs/webrev-hex-encoder-8251989

Webrev for applying to java.security:
    http://cr.openjdk.java.net/~rriggs/webrev-hex-security-8252055

CSR:
    https://bugs.openjdk.java.net/browse/JDK-8251991

Issue for API and a few uses:
    https://bugs.openjdk.java.net/browse/JDK-8251989

Issue for Use in java.security and tests:
    https://bugs.openjdk.java.net/browse/JDK-8252055