Added new EmptyArray function.

[cgranade]

This PR adds a new EmptyArray function to help deprecate the new keyword, and in keeping with the improvements at microsoft/QuantumLibraries#313.

[msoeken]

Looks great to me. Thanks!

[msoeken]

I was wondering what happens if we call Default<'T[]>() for some 'T? Would that also return an empty array?

[cgranade]

See microsoft/iqsharp#325 for a discussion of build failures.

[cgranade]

I was wondering what happens if we call Default<'T[]>() for some 'T? Would that also return an empty array?

At the moment, there's a bug that causes Default<'T[]>() to return a null array (which isn't even a concept in Q#) when run against the simulation runtime. More generally, though, this new function could allow us to deprecate Default entirely in favor of using array functions like EmptyArray, ConstantArray, and so forth. See further discussion at microsoft/qsharp-compiler#589.