Safer traces, part 1: Provider builder

[daladim]

This PR depends on #53 and #55 (as the first few commits are cherry-picked from these branches), that must be merged first.

This introduces a ProviderBuilder that must be used to create Providers. This way:

• setters can be removed from Provider, which will ensure it is not modified at runtime (this will help reviewing unsafe blocks and resolving #45 )
• the compiler now enforces GUID are set. No need to check it at runtime

[daladim]

I've rebased and resolved the git conflicts :)

I'm not sure why docs.rs was unable to generate the documentation for the 0.1 version, but it should be fine now, at least for version 1.0.

Feel free to upload it to your website, but I think it is too early to release a 1.0 (and docs.rs will probably refuse to build a doc for a un-released version...unless you can try with a some -beta or something). I'd like to fix #45 first so that the 1.0 version is polished when it gets out :)

You may want to release a 0.1.2 however with the current content of master, that includes a few little fixes over 0.1.1 :)

[daladim]

And I think I'll be able to create a PR that fixes #45 tomorrow or on Friday. It's sitting on my PC already, but that's a matter of rebasing, fixing the conflicts and making sure the doc still makes sense :)