mirrors now carry a class tag of the receiver, so that they can detect
value classes being reflected upon and adjust accordingly (e.g. allow
Int_+ for ints, but disallow it for Integers).

Surprisingly enough derived value classes (SIP-15 guys that inherit from AnyVal)
have been working all along, so no modification were required to fix them.

Arguments provided in by-name positions are now automatically wrapped
in Function0 instances by method mirrors.

In 911bbc4 I've completely overlooked the fact that
reflectConstructor exists and that is also needs sanity checks.

Now reflectConstructor checks that the incoming symbol is actually a ctor,
and that it is actually a ctor of the class reflected by the current mirror.

Previously `checkMemberOf` was blocking base fields and methods
that are overriden in receiver.getClass. Now this is fixed.

The fix also uncovered an issue with field mirrors. Currently
their `get` and `set` methods don't respect overriding and always
return field values from a base class.

After discussing this on a reflection meeting, we decided that this
behavior is desirable and that for overriding people should use
reflectMethod and then apply on getters/setters. See the discussion at:
scala#1054.