change + mode to pass by-value

[nikomatsakis]

Right now we are using + as the "by value" mode, but in fact at runtime it passes by reference. We should change it to pass by value.

[nikomatsakis]

assigned to 0.4 milestone as "mode removal" is a big goal

[bblum]

will this affect the semantics of passing @ boxes in +-mode? i wrote TLS with this in mind, and am not sure whether or not it will break if changed.

[nikomatsakis]

this would not affect the semantics, I don't think.

[graydon]

I think this is possibly-done at this point, but someone should confirm.

[nikomatsakis]

This is done.