[flexi-streams-announce] New release 0.15.0

[Edi Weitz]

Too much changes to mention them individually.  This started out as
making the string functions faster and ended in a major redesign.  The
new release also fixes some bugs, adds a few new features, is slightly
incompatible with previous versions, and likely breaks a few things.
Check it out for yourself (and keep a copy of the latest release just
in case).

Thanks to Hans Hübner for his work on FLEXI-STREAM's performance.