Turns out the world is not well-behaved and and I get ambigous operators from
the conversion-to-value_type and conversion-to-any-vectorclass-vector
operators.

The first one we need to be usable as the type we are proxying, obviously.
The second one we need so a vectorclass vector can be broadcast-assigned or
broadcast-copyconstructed from a proxy.

So now I have to resolve all the ambiguities by providing all operators
anyway.