New function for string and array libraries and minor bug fixes

Changelog

* new string_ci_get_between()
* new string_get_between_ex()
* new array_device_index()
* new array_integer_index()
* new array_double_index()
* new array_string_index()
* new array_function_deprecated()
* fixed bug in string_size_error(), now it returns '' instead of 'error'
* new test_array suite
* new test_string suite