Rustpkg docs and tests

[catamorphism]

r? @graydon - This pull request continues to sketch out the test cases, this time hopefully with the right directory structure; and adds a sketch of a manual for rustpkg.

[steveklabnik]

I only read the docs but I like them. :)

[graydon]

I'd use "workspace" rather than "package source" here; too confusable with the src/ subdir and/or the place you downloaded the package from.

[brson]

This is looking good. Thanks.

[asb]

@catamorphism It strikes me that rustpkg.md discusses conventions for bench, test, lib and bin but says nothing about documentation. I know API docs are likely to be extracted using rustdoc, but many projects may (I'm an optimist!) include tutorial-style introductions and the like, or at least a README and LICENSE/COPYING file. Presumably it would make sense to install these.

I agree with @graydon that 'workspace' makes more sense than 'package source' (which to me suggests the upstream git repo, which you'd expect wouldn't contain a 'build' dir).

[brson]

+1 to teaching rustpkg about docs

[catamorphism]

I will incorporate these comments and then push a new commit to this branch.

[catamorphism]

@asb : Good suggestion, but beyond the scope of this pull request. I filed a separate issue, #6005.

[catamorphism]

So as to preserve comments, I'll actually just open a new pull request.