<10% slower, 100% idiomatic

```
[info] Benchmark                                       Mode  Samples        Score  Score error  Units
[info] r.VecMapBenchmark.squareDoubleArrayWithLoop    thrpt       20  1478640.429    78967.084  ops/s
[info] r.VecMapBenchmark.squareDoubleArrayWithMap     thrpt       20    71711.934     3269.255  ops/s
[info] r.VecMapBenchmark.squareDoubleVec              thrpt       20  1397215.415    17459.019  ops/s
```

Ping @tixxit: I decided to use the name `MbArray` instead of `@local Array`
since `@local Array` would break Scala's Array invariants and it shouldn't
be allowed to escape at all. `MbArray` on the other hand, being a wrapper
around an array, can escape at will. It may become suboptimal, but that's
what the warnings are for. :)

Btw, those nasty miniboxing flags "-P:minibox:warn" and
"-P:minibox:Yrewire-functionX-application" will become standard when
I finally release version 0.4, so don't worry about them for now.