Support constructing cc_std::std::optional payloads in place.

[copybara-service]

Support constructing cc_std::std::optional payloads in place.

std::optional<T> can hold a T which cannot be produced by value in Rust,
most importantly the layout-compatible cc_std::std::string, which is !Unpin
and only constructible through a Ctor. Previously an optional<T> could only
be built from an owned T, via optional::new or From<Option<T>>, so such a
payload was unreachable.

Add in-place construction and mutation:

• optional::ctor_some and optional::ctor_nullopt return Ctors which
  construct the payload directly into the optional's storage.
• optional::emplace and optional::reset are the pinned counterparts to
  assigning through as_mut and to take. They construct and destroy the
  payload where it lives rather than moving it, so they remain available when
  the payload is pinned.

ctor_some is an inherent function rather than a CtorNew impl because ctor
provides a blanket impl<T: Default> CtorNew<()> for T, which an
impl<T, C: Ctor<Output = T>> CtorNew<C> for optional<T> would overlap.

SomeCtor::ctor clears engaged before running the payload's constructor, so a
payload constructor which fails or panics leaves behind a valid empty optional
rather than one whose destructor would read an uninitialized payload. Drop now
shares that logic, which also makes it safe against a panicking payload
destructor.

trivial_optional is deliberately unchanged: it is the destructor-free tier
used for fields of Copy structs, and a payload which is !Unpin or
ctor-only is never Copy.