zfec-1.5.2 released, fixing windows+tahoe

[Brian Warner]

zfec-1.5.2 is now available. It turns out that Appveyor (our Windows CI
tool) is using a pretty old C compiler, or at least we're telling it to
do something that uses one without C99 syntax. The recent improvements
to zfec's C code were breaking under that compiler, causing Tahoe builds
to fail.

I've reverted the syntax changes, and turned on appveyor testing for
zfec so we can spot this sort of thing in the future.

cheers,
 -Brian