ARROW-1336: [C++] Add arrow::schema factory function, simply some awkward constructors

[wesm]

No description provided.

[wesm]

cc @cpcloud

I'm not sure why Appveyor is upset

[pcmoritz]

Have you considered naming it make_schema? That'd go well with std::make_unique, std::make_shared, std::make_pair, etc; then you can also get rid of ::arrow:: (I assume that was to avoid name clashes).

[cpcloud]

LGTM

[wesm]

@pcmoritz I did think of that; this is more consistent with functions like arrow::field, arrow::timestamp, etc. I would rather change everything than do something inconsistent, so perhaps we can revisit if there are strong feelings about the naming style of these factory functions

I wonder what is going on with the segfault in Travis CI, we should try to figure that out (I have seen it before): opned ARROW-1338