Implement concatenation on new vector types

[graydon]

Residual task from #2112. Now that we have operator overloading we can probably move all this to the vec module and remove those translation paths from the compiler entirely. But currently concatenating []/&, []/_, []/@, []/~ don't work.

[eholk]

Would these return new vectors (the [] kind), or the same type as their inputs?

[msullivan]

This is subsumed by the more general #2921.