Add support for ..base on static struct initializers.

[kevinmehall]

With an expression like

static w : foo = foo { a:5, ..x };

Rust currently gives the error "constant contains unimplemented expression type". This branch implements support for constant structs with ..base.

[alexcrichton]

For completeness, could you add some negative tests as well to compile-fail? Something like expanding struct X into struct Y or an integer into struct X or something like that.

[kevinmehall]

I rebased the commit to add compile-fail tests. There weren't similar tests for non-static struct expressions, so I also included those, though I think they might use the same code path.