FTL: Refine container semantics

Allow constructing StaticVector and SmallVector by moving smaller
convertible vectors, which so far incurred a copy. Allow the same
copy/move conversion for SmallMap.

Consistently with StaticVector, do not require assignable elements for
the SmallVector to be assignable, which notably enables SmallMap to be
assignable despite its const keys.

Allow comparison of convertible containers.

Bug: 185536303
Test: ftl_test
Change-Id: I35923e794ef26178dc3072f514dea7ad5600bc15