Hi

Introduction
------------
Inspired by Ricardos commit here I rewrote most of the npm importer.

Added memoization, receive, stream->list, values and rewrote the tarball
fetcher to use only npm-uri and tarballs from the registry. Additionally
I implemented handling of scoped packages (e.g. @babel/core).

It contains less lines of code than Jelles importer.

The single import works and is a lot faster and more reliable than
before when fuzzy matching on github was used. See it in action:
http://paste.debian.net/1054384/

Caveats:
1) we don't know if the registry-tarballs are reproducible.
2) filename is the same as the upstream tarball -> we should convert it
to guix-name.
3) we have to download the tarball because sha256 is not among the
hashes computed by npm. (I emailed npm@npmjs.org to ask for them to
compute it for all their tarballs :) )

Help wanted
-----------

There is a bug which only affects the recursive importer. I tried hard
finding it but I'm in way over my head and my guile-foo does not seem to
cut it with this one. :) 

For recursive output it downloads but outputs #f in the end instead of
the sexps. See example output: http://paste.debian.net/1054383/

Trying to debug via the REPL I met this:
scheme@(guile-user) [1]> (npm-recursive-import "async")
$3 = #<stream ? ...>

Any ideas? 

-- 
Cheers 
Swedebugia