ctf: Cleanup StructDeclaration

ctf: Cleanup StructDeclaration

No need to keep separate lists for the fields and values of structs.
A LinkedHashMap preserves the insertion order, and Iterable defines
a specific iteration order.

No apparent performance impact, both versions hover around 975-980 ns/ev
on my machine.

Change-Id: I487b2c756d5827a7489a2ef65e550fb3d018cb72
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/29009
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Tested-by: Hudson CI