Remove BLI_str_utf8_as_unicode_and_size and
BLI_str_utf8_as_unicode_and_size_safe.
Use BLI_str_utf8_as_unicode_step instead since it takes
a buffer bounds argument to prevent buffer over-reading.
This adds a generic string search library in `BLI_string_search.h`.
The library has a simple to use C api that allows it's users to
filter and sort a set of possible search results based on some
query string.
Reviewers: Severin
Differential Revision: https://developer.blender.org/D8825
