First version of FFIStructFieldSlot (WIP)

[tinchodias]

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[tinchodias]

There is a key difference: instead of double-dispatching on the types to generate the accessors at "rebuild accessors" time, in this version the types are kept live, pointed by the slots and #basicHandle:at:put: and #basicHandle:at: are sent by the slot on each read and write.

[tinchodias]

The tests re copied and adapted from another UnifiedFFI test suite.
My image was broken so I just commit the code to run in another fresh image in the future.
So far, this was my test:

source := SDL_Point.

target := source duplicateClassWithNewName: (source name, UUID new asString36) asSymbol.
target convertToFFIFieldSlots.

instance := target new.
instance x: 12.
instance y: 17.

self assert: (target slots collect: [ :each | each read: instance ]) = #(12 17).

"Tear down:"
instance := nil.
target removeFromSystem. target := nil.