Define [com_interface] IID as string

[Rantanen]

Define the GUID as a string in the #[com_interface("<iid>")] attributes.

Needed something easy and simple to work on. :)

Closes #90

[Rantanen]

The CI is private by the way. I forgot to run rustfmt on the changes, which resulted in a failing CI build, but all I get is access denied when trying to access it:
https://dev.azure.com/microsoft-rust/332945ff-0d35-4cde-b1e2-30e171b5154d/_build/results?buildId=353

[rylev]

@Rantanen awesome - thanks! Any way we could add a compile check to make sure we get a nice error message when not using a string literal?

[Rantanen]

Added that test and rebased on top of the master branch to resolve conflict in progress.rs and to fix the path tests, which need to use string GUIDs in this branch.

[Rantanen]

I don't really know what is causing the build failure here. I've tried replicating the .azure-pipelines.yml build locally and I can't get it to fail. I'll try fixing the timestamp on the last commit, which is at least confusing GitHub (since due to rebasing things, it's timestamp is before the second last commit).

[Rantanen]

There was an error in the tests. Apparently I had mixed the old pre-string path tests and this one when verifying that the errors were correct. Instead the new test failed to compile due to missing use and now because of non-string GUID parameter.

Fixed that and now the CI is green as well. Still no idea what the original issue was though. :|