Add MScalar Type

[willow-ahrens]

Why not also have an MScalar type? This also includes the zero dimensional broadcast fix from pja/zero-dim-broadcast. Perhaps this would help with #580?

[tkoolen]

This appears to introduce an ambiguity on 1.0.3. Do you still care about this?

[timholy]

Also worth pointing out #518

[willow-ahrens]

While I think these changes might provide some nice symmetry with the "Scalar" type, I mostly wanted to fix indexing for zero-dimensional MArrays, which I think happened in #581.

[andyferris]

I suppose we could add this, for symmetry as noted. On the other hand, many use cases are well covered by RefValue or the immutable Scalar - which, really, only existed to help me do some broadcasting tricks (does Base recommends Ref for that now?).

(On a side note, I'm still curious whether something will shake out of the references work (that Keno is interested in) that covers the broadcast-as-a-scalar case, too)

[c42f]

I think we should close this and direct people to use Refs or 0-dimensional MArrays if that covers the desired use cases. (I'm somewhat inclined to deprecate Scalar for similar reasons.)

[andyferris]

Yes, it's tempting to do as Chris says and just leave this.

(I always hoped for an alias to appear in Base for Array{T,0}, which we would then mimic, but there has been no successful argument for that yet).

[c42f]

Ok, I'll close this for now. However, if you do find a need for MScalar which is not covered by the existing types I'm completely happy to reconsider.