Speed up base32::decode()
Speed up base32::decode()

From 1.03% to 0.19% of the runtime of 'nix-instantiate "<nixpkgs>" -A
texlive.combined.scheme-full --dry-run'.

pikajude/nix,ehmry/nix,NixOS/nix,ehmry/nix,ehmry/nix,rycee/nix,rycee/nix,rycee/nix,ehmry/nix,NixOS/nix,pikajude/nix,rycee/nix,ehmry/nix,NixOS/nix,NixOS/nix,NixOS/nix,ehmry/nix,pikajude/nix,rycee/nix