[C6] Transducers v0.9.0 and C6 support

Hey all, I wanted to announce that transducers is now at v0.9.0 and has 
CHICKEN 6 support!

I've run test-new-egg, and this is what I got:

==== transducers (1 of 1) ====
   Fetching........................................[ ok ] 0s
   Reading .egg....................................[ ok ] 0s
   Checking dependencies...........................[ ok ] 0s
   Checking category...............................[ ok ] 0s
   Checking license................................[ ok ] 0s
   Checking author.................................[ ok ] 0s
   Installing......................................[ ok ] 1m51s
   Checking version................................[ -- ]
   Testing.........................................[ ok ] 9s
   Checking documentation..........................[fail] 0s

Of course, I need to create that new page on the wiki for C6, but 
otherwise looks all above board! The URL for the release-info file is:

<https://gitlab.com/ThatGeoGuy/chicken-transducers/-/raw/main/transducers.release-info?ref_type=heads>

Unfortunately, I also am announcing that as part of porting transducers 
to C6, I have removed the (transducers mappings) module from the egg. I 
made this decision because this part of the egg concerns the only code 
that relies on external datatypes to function. It pulled in a lot of 
dependencies, and it was blocking C6 support.

Regardless, if there is any desire to bring back transducers-mappings, 
I'd be happy to do so under the umbrella of a separate egg, rather than 
bundling it into the core transducers egg.

I'll be updating the docs for C5 and C6 shortly.

Cheers,
-- 
Jeremy Steward