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/Cargo.lock

@@ -473,6 +473,12 @@ dependencies = [
  "syn 1.0.103",
 ]
 
+[[package]]
+name = "data-encoding"
+version = "2.3.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "23d8666cb01533c39dde32bcbab8e227b4ed6679b2c925eba05feabea39508fb"
+
 [[package]]
 name = "derivation"
 version = "0.1.0"
@@ -1785,6 +1791,28 @@ dependencies = [
  "winapi-util",
 ]
 
+[[package]]
+name = "test-case"
+version = "2.2.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "21d6cf5a7dffb3f9dceec8e6b8ca528d9bd71d36c9f074defb548ce161f598c0"
+dependencies = [
+ "test-case-macros",
+]
+
+[[package]]
+name = "test-case-macros"
+version = "2.2.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e45b7bf6e19353ddd832745c8fcf77a17a93171df7151187f26623f2b75b5b26"
+dependencies = [
+ "cfg-if",
+ "proc-macro-error",
+ "proc-macro2 1.0.47",
+ "quote 1.0.21",
+ "syn 1.0.103",
+]
+
 [[package]]
 name = "test-generator"
 version = "0.3.0"
@@ -2116,9 +2144,11 @@ version = "0.1.0"
 dependencies = [
  "anyhow",
  "blake3",
+ "data-encoding",
  "lazy_static",
  "prost",
  "prost-build",
+ "test-case",
  "thiserror",
  "tonic",
  "tonic-build",