Graham/fix nix output parsing

[grahamc]

The code in #133 is pretty good, check out these proposed patches for some feedback / suggestions

This borrows upstream Rust code to implement a Lines iterator which produces OsStrings, and replaces the parse_nix_output code to use it. From the commit where I drop that function:

nix: drop parse_nix_output

Prefer the iterator format, especially when we're able to
use familiar forms of passing functions around.

In this case the parse_nix_output function probably shouldn't
skip newlines.

Additionally, reducing the powerful model of iterators down
to only supporting (and actually requiring) a single function
isn't doing us much favor.

Finally, in the tests for the builder I add a proof-positive test to assert that the bytes come through and that the shell derivation is produced.