std/arrays: new module for dealing with arrays

[DeedleFake]

There should be an array module in the standard library for dealing with arrays in various ways. Specifically, it should have support for at least the following:

• [x] ~Accessing a specific index.~ Dropped in favor of at in std. See #72.
• [x] ~Getting the length of an array.~ Dropped in favor of len in std. See #72.
• [x] Sorting.
• [ ] Run an array as if it was a compound.

[DeedleFake]

May also want to move the creation of streams from arrays from the stream module to this one.

[DeedleFake]

A few more:

• [x] concat. Alternatively, just stick this in + in std along with string concatenation.
• [x] ~append. Similar to append() in Go. Need to make sure that slicing changes the capacity as well to make sure that this doesn't modify parts of existing slices.~ Dropped in favor of concat. See #122.

[DeedleFake]

Even more:

• [ ] streamReverse. Streams the array from the last index to the first.