feat(tvix/store): add nixbase32 mod

This implements the nix-specific base32 encoding and decoding, exposing
a subset of the API that the data-encoding crate provides.

Nix uses a custom alphabet, no padding, and encodes bytes in reverse
order. The latter one is the reason we can't just use the data-encoding
crate directly.

Three odd corner case tests ported over from go-nix failed. We opened
b/235 to further investigate.

Change-Id: I73fab6ddd67177d882e4c3f2b48761c95853d558
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7683
Reviewed-by: tazjin <tazjin@tvl.su>
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/store/src/main.rs

@@ -1,3 +1,4 @@
+mod nixbase32;
 mod proto;
 
 #[cfg(test)]