Conversions of ext_vector types

Hi Kevin,

I've looked, but not found the story behind this. Would there be
objections to having conversions, like other ext_vector operations,
perform the corresponding scalar operation elementwise instead?

Here's some history for you:
http://lists.cs.uiuc.edu/pipermail/cfe-dev/2010-April/008844.html
http://lists.cs.uiuc.edu/pipermail/cfe-dev/2010-April/008845.html

Cheers,
Anton.