implement fetchurl (as a copy of fetchTarball)

[wavewave]

implementation of fetchurl.
I found the implementation is exactly the same as fetchTarball now.

[wavewave]

Before

$ dist/build/hnix/hnix --eval --expr 'builtins.fetchurl https://raw.githubusercontent.com/haskell-nix/hnix/master/LICENSE'
hnix: Could not look up attribute fetchurl in "an attr set"

and
After

$ dist/build/hnix/hnix --eval --expr 'builtins.fetchurl https://raw.githubusercontent.com/haskell-nix/hnix/master/LICENSE'
"/nix/store/2crhdsmnw0iyjzmzca3qkzd0vp8ri5jb-LICENSE"

$ cat /nix/store/2crhdsmnw0iyjzmzca3qkzd0vp8ri5jb-LICENSE 
Copyright (c) 2014, John Wiegley All rights reserved.

Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are met:
...

[wavewave]

this is not a real implementation. will work on it correctly.