Review and update noexcept status about value_types

[Twon]

Nothing else, I was just waiting for a review of #59 before closing

[Twon]

Example of conditionally noexcept method for sdt::unique_ptr: https://eel.is/c++draft/unique.ptr.single.observers

However, this is one of the few examples which could be relevant. This exists because the pointer type within a std::unique_ptr can be specialised via way of the deleter type: https://stackoverflow.com/a/31694199/4764531. This is not applicable to indirect or polymorphic

[Twon]

PR #59 now contains 3 conditional noexcept methods for each of indirect and polymorphic. This lines up with the few usages of conditional noexcept already in the standard. For instance, as pointed out by Thomas Russell, std::vector::swap has conditional noexcept specifications: https://eel.is/c++draft/vector#capacity

[jbcoe]

What do we need to do?