Vector2 constructor benchmark

[spydon]

I worry that these changes may confuse the (somewhat brittle) analysis that allow the length of _v3storage to sometimes become known in the optimizer.

Due to this comment #294 (comment) I wanted to make sure that it wasn't slower to create Vector2 objects with named constructors than with "redirecting" factory constructors. The benchmark shows no sign of it being slower than before.

@rakudrama is there anything I should change, or add, in the benchmark?

[coveralls]

coverage: 26.369%. remained the same when pulling 07d97b0 on spydon:vector2_benchmark into a3aca09 on google:master.