Contiguous vector storage

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Depends on https://github.com/mattt/Surge/pull/110.

Addresses https://github.com/mattt/Surge/pull/110#discussion_r315266193.

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Apple's documentation for ContiguousArray states:

The ContiguousArray type is a specialized array that always stores its elements in a contiguous region of memory. This contrasts with Array, which can store its elements in either a contiguous region of memory or an NSArray instance if its Element type is a class or @objc protocol.

If your array’s Element type is a class or @objc protocol and you do not need to bridge the array to NSArray or pass the array to Objective-C APIs, using ContiguousArray may be more efficient and have more predictable performance than Array. If the array’s Element type is a struct or enumeration, Array and ContiguousArray should have similar efficiency.

For more information about using arrays, see Array and ArraySlice, with which ContiguousArray shares most properties and methods.

Source (emphasis mine)

What this boils down to is that for value types (and Surge's Scalar are value types, primitives even) the internal memory representation of Array and ContiguousArray should be the same, i.e. contiguous.

I'm gonna close this as we're not going to get any performance improvements from it and thus the more common type Array is preferable over the less well known ContiguousArray.