type_caster<std::optional>: support non-assignable types

[WKarel]

So far, type_caster<std::optional>::from_python requires T to support copy-assignment, because it uses std::optional::operator=, which in turn uses

• in-place construction of T if empty, or
• copy-assignment of T if non-empty.

Removed this requirement by replacing std::optional::operator= with std::optional::emplace, which always uses in-place construction.

Apart from supporting non-assignable types, there should be no changes in behaviour, because type_caster<std::optional>::from_python is called only once on each instance, and so its member value is always empty, anyway.

My use case: bind functions with arguments of type std::optional<Eigen::Map>. Compilation fails without this change.

[wjakob]

Nice, thank you!