Introduce provision selection and prevent overriding mounted provisions

[Abnaxos]

The original plan for handling duplicate provisions has a major flaw. So major, that currently, mounted provisions will simply break things.

Let's say we have two features featureA and featureB, that both provide a foo. Currently, the compiler will report an error and require the user to override the provision in the configuration, something like this:

@Provision
public Foo foo() {
  return featureA().foo();
}

However, in the generated code, the mounted provisions of both featureA() and featureB() will be overridden to use the provision from the configuration → we get an infinite recursion.

Introduce some attributes to the mount annotation:

@Provision.Mount(select="..." /*, reject="..." -- don't add this, see #104*/)
public abstract Foo foo();

This gives the compiler all the information it needs to correctly generate the code. It's also more visible what's actually going on.

Related to #104

[Abnaxos]

Solved with #104. No provision selection though, but that's fine.