Add readArrayOf:size: method in ExternalAddress

[tinchodias]

Added method to read an array of a specified type and size.

I should add a test.

[tinchodias]

Note: this method wasn't useful the read the SDL_DisplayMode** result of this function because I wanted an array of SDL_DisplayMode instead of an array of pointers to SDL_DisplayMode.

I wrote this other method for that case:

readStructArrayFromPointers: aFFIStructure size: count
	"Return an array of structs of the specified size"

	^ Array streamContents: [ :stream |
		| address |
		address := self.
		1 to: count do: [ :index |
			stream nextPut: (aFFIStructure fromHandle: (address pointerAt: 1)).
			address := address + FFIExternalType pointerSize ] ]

[tinchodias]

I wonder if this if is right. I kept it from readArrayOf:until:, which was the base.