Revert "Replace boost::aligned_storage with alignas std::array"
This reverts commit 32ff87f7f6.
This causes issues on macOS/clang
```
../libs/pbd/pbd/stack_allocator.h:152:53: note: destructor of 'StackAllocator<std::__tree_node<std::__value_type<ARDOUR::DataType, std::map<unsigned int, unsigned int, std::less<unsigned int>, PBD::StackAllocator<std::pair<const unsigned int, unsigned int>, 16>>>, void *>, 2>' is implicitly deleted because field '_buf' has a deleted destructor
152 | alignas(16) std::array<value_type, stack_capacity> _buf;
```
4a99026cc9