Add fetchMercurial primop

E.g.

  $ nix eval '(fetchMercurial https://www.mercurial-scm.org/repo/hello)'
  { branch = "default"; outPath = "/nix/store/alvb9y1kfz42bjishqmyy3pphnrh1pfa-source"; rev = "82e55d328c8ca4ee16520036c0aaace03a5beb65"; revCount = 1; shortRev = "82e55d328c8c"; }

  $ nix eval '(fetchMercurial { url = https://www.mercurial-scm.org/repo/hello; rev = "0a04b987be5ae354b710cefeba0e2d9de7ad41a9"; })'
  { branch = "default"; outPath = "/nix/store/alvb9y1kfz42bjishqmyy3pphnrh1pfa-source"; rev = "0a04b987be5ae354b710cefeba0e2d9de7ad41a9"; revCount = 0; shortRev = "0a04b987be5a"; }

  $ nix eval '(fetchMercurial /tmp/unclean-hg-tree)'
  { branch = "default"; outPath = "/nix/store/cm750cdw1x8wfpm3jq7mz09r30l9r024-source"; rev = "0000000000000000000000000000000000000000"; revCount = 0; shortRev = "000000000000"; }