Allow allocating memory for non-trivially-destructible types

[pknowlesnv]

The allocator isn't in charge of destructing the object, so this would be safe. It's only the code in decodeless/allocator_construction.hpp that is unsafe for non-trivially-destructible types. With this change, the linear allocator could be used as a regular memory pool with STL objects.

Adding a "stack" memory resource would be handy too! This could trivially just be struct { std::byte array[size]; }.