Function to count number of substring occurrences in the string:
- in case of needle is multi char - counts non-intersecting substrings
- the code is based on position helpers.
The following new functions is available:
- countSubstrings()
- countSubstringsCaseInsensitive()
- countSubstringsCaseInsensitiveUTF8()
v0: substringCount()
v2:
- add substringCountCaseInsensitiveUTF8
- improve tests
- fix coding style issues
- fix multichar needle
v3: rename to countSubstrings (by analogy with countEqual())
