Clean up Unicode function names

#1938670: Convert unicode.inc to \Drupal\Component\Utility\Unicode comes with some PHP function wrappers. Unicode::truncate(), Unicode::mimeHeaderEncode(), etc. Some of them arn't living under the naming conventions:

• Unicode::ucfirst()
• Unicode::substr()

Although it is nice to have PHP wrappers be similarly named to the PHP functions, it's even nicer to have consistent coding standards.

Possible solutions:

1. Fix the case of the functions: Unicode::ucFirst(), Unicode::subStr(), etc.
2. Rename the functions entirely: Unicode::upperCaseFirst(), Unicode::subString(), etc.
3. Leave as is and live with the broken coding standards