[Toybox] xzcat initial implementation (based on xz-embedded)

[Isaac Dunham]

Hello,
This is the code we were discussing, ready to apply.

I've made some changes (merging a couple headers and the CRC implementations, removing a few definitions that were meant for compatability with use in the kernel, plus what was necessary for using the toybox infrastructure) and enabled all the BCJ filters, but it's mostly the same as upstream xz-embedded.
Although I put this in toys/pending and disabled it by default, I have verified that it builds without warning and is functional.  
It does not accept a filename, so you need to use redirection.

Toybox with only this enabled is ~19k shared/glibc, ~34k static/musl.

-- 
Isaac Dunham <idunham at lavabit.com>
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