FTL: Generalize StaticVector in-place construction

Replace the std::in_place_type constructor, which is limited to one
argument per element, with construction from InitializerList, which
forwards argument tuples for each element.

SmallMap will use this syntax for its key-value pair constructor.

Bug: 160012986
Test: ftl_test
Change-Id: I9331f20268532cb09e980475d68768ba891aca1f