Add Linker and Scope toString + tests

[TWiStErRob]

Fixes:

reflect\src\main\java\dagger\reflect\Scope.java:84:
warning: [ObjectToString] Linker is final and does not override Object.toString, so converting it to a string will print its identity (e.g. `Linker@ 4488aabb`) instead of useful information
.
            "Unable to find binding for key=" + key + " with linker=" + linker);
                                                                        ^
    (see https://errorprone.info/bugpattern/ObjectToString)