Updated implementation (#8)

* replace iterator with plain pointer and add tests

* bump to php 7.4

* add github actions

* update phpunit config

* allow new ascii-table package

* remove phpdox from readme

Merge pull request #7 from IlyaSerdyuk/master

Add support PHP 7.4

Merge pull request #5 from dmryabov/remove-hoa-ustring

remove hoa/ustring dependency

Remove check on empty string

Merge pull request #2 from LTD-Beget/unicode

unicode

edit readme