RFR: 8251989: Hex formatting and parsing utility

[Roger Riggs]

java.util.HexFormat utility:

 - Format and parse hexadecimal strings, with parameters for delimiter, prefix, suffix and upper/lowercase
 - Static factories and builder methods to create HexFormat copies with modified parameters.
 - Consistent naming of methods for conversion of byte arrays to formatted strings and back: formatHex and parseHex
 - Consistent naming of methods for conversion of primitive types: toHexDigits... and fromHexDigits...
 - Prefix and suffixes now apply to each formatted value, not the string as a whole
 - Using java.util.Appendable as a target for buffered conversions so output to Writers and PrintStreams
   like System.out are supported in addition to StringBuilder. (IOExceptions are converted to unchecked exceptions)
 - Immutable and thread safe, a "value-based" class

See the [HexFormat
javadoc](http://cr.openjdk.java.net/~rriggs/8251989-hex-formatter/java.base/java/util/HexFormat.html) for details.

Review comments and suggestions welcome.

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Commit messages:
 - 8251989: Hex formatting and parsing utility

Changes: https://git.openjdk.java.net/jdk/pull/482/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=482&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8251989
  Stats: 1665 lines in 12 files changed: 1503 ins; 144 del; 18 mod
  Patch: https://git.openjdk.java.net/jdk/pull/482.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/482/head:pull/482

PR: https://git.openjdk.java.net/jdk/pull/482