avoid placement new

The placement new pattern used throughout these files was flagged by
GCC-16 with -Wuninitialized and -Wmaybe-uninitialized.  Placement new
does, technically, result in an uninitialized value even though the
intent was to cast the pointer onto existing storage.  Using
`start_lifetime_as` instead better expresses the intent and avoid the
GCC-16 warnings.

GCC-15 doesn't provide `std::start_lifetime_as` yet, so provide it
in a polyfill for backwards compatibility.

There are a large number of `reinterpret_cast` calls already done in
this repository that likely should be converted to `start_lifetime_as`
to avoid undefined-behavior in later versions of GCC.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I227764f1191bbc70105200a703106c8d21e5e5eb