There are many combinations of `append(Integer.toHexString(i))` and
`append(Long.toHexString(i))` in the JDK code. I request to add method
`appendHex(int)` and `appendHex(int)` to StringBuilder and StringBuffer. This
can reduce duplicate code within the JDK and will also be useful to users.
I submitted a PR ( https://github.com/openjdk/jdk/pull/15998
<https://github.com/openjdk/jdk/pull/15998 > ), including the code and tests
for the new method. I also replaced the code that uses the append + toHexString
combination inside the JDK with appendHex.
Please review and don't hesitate to critique my approach and patch.
