feat(tvix/eval): implement builtins.hashString

Implements md5, sha1, sha256 and sha512 using the related crates from
the RustCrypto hashes project (https://github.com/RustCrypto/hashes)

Change-Id: I00730dea44ec9ef85309edc27addab0ae88814b8
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11005
Tested-by: BuildkiteCI
Reviewed-by: aspen <root@gws.fyi>

tvix/eval/src/tests/nix_tests/eval-okay-groupBy.nix

@@ -1,4 +1,4 @@
-with import ./../lib.nix;
+with import ./lib.nix;
 
 builtins.groupBy (n:
   builtins.substring 0 1 (builtins.hashString "sha256" (toString n))