Define X_n variables on entry to function generated by nsplat/ngenerate

[simonster]

This appears to fix DataArrays. cc @timholy

[timholy]

LGTM. As you surely know, you can also use the @nexprs N (d->X_d = ...) construct, but given that you have to splice in the symbol your version is probably easier. Merge at will. (And then we should tag a new release.)

[simonster]

Hmm, it seems like @nexprs might be a better idea if X is a heterogeneously typed tuple, since it looks like X_1, X_2 = X doesn't give us proper type information.

[timholy]

Nice. I hadn't remembered that distinction, but it makes sense.

[simonster]

Thanks for reviewing!