Change vendor / namespace to graphp

[clue]

This depends on #130 first, as we need to decide on a proper package/repository name first and then move the repository.

One the repository is moved, we should change our vendor namespace accordingly. This will result in a BC break that is relatively easy to deal with.

We also need to update our Packagist vendor name to "graphp". Because quite a few repositories still reference the old package name, both packages need to coexist (for quite some time). We will need to set up a new package there and deprecate the old package name. Among others, this means that download counts will be reset to zero and users will have to make an explicit change in the package name. See composer/packagist#47 for some background.

Also keep in mind that we need to update our own components. Given they will depend on a whole new package, it's probably safe to consider this a BC break for each.

[clue]

Closed via #166