Use built-in functions for ascii85 and asciihex

[dhdaines]

Pull request

Fixes #602 by using built-in ascii85 and adds bonus built-in asciihex ... Maybe they are faster?

How Has This Been Tested?

Kept the unittests to make sure these functions do the same thing as our old versions.

Checklist

• [x ] I have read CONTRIBUTING.md.
• [x ] I have added a concise human-readable description of the change to CHANGELOG.md.
• [x ] I have tested that this fix is effective or that this feature works.
• [x ] I have added docstrings to newly created methods and classes.
• [x ] I have updated the README.md and the readthedocs documentation. Or verified that this is not necessary.