* Improve Allocator handling
- Templatize Allocator on vector_downward, and make it own the allocator
instance so it can manage lifetimes.
- Templatize + rename FlatBufferBuilderT accordingly, and add a typedef
to FlatBufferBuilder so old code continues to work.
- Fix some issues with the release deleter
- More details in github issue #4311
* Fix constexpr for older MSVC
* Reimplement allocator improvements via inheritance
Instead of templates, use an abstract base class and some unique_ptrs to
implement the new and improved allocator.
* Fix misplaced newline
* Add missing override keyword
* Add macro for func delete to support older compilers
* Explicitly move BufferDeleter (maybe fixes VS 10?)
* Revert previous attempt at VS10 fix
* Try yet another workaround for MS 10
* Use FLATBUFFERS_NOEXCEPT macro
* Add FLATBUFFERS_OVERRIDE macro
* Fix some issues with MSVC 16 hack
* Remove dep on unique_ptr
* Update DEPRECATED macro with a message
* Sigh, no delegating ctors before c++11
* Fix simple_allocator stub
* Relax FlatBufferBuilder ctor
* Add vector_downward reset + assert in clear
* Rename DetachedBuffer to FlatBuffer + remove unnecessary deprecations
* Add flatbuffers::FlatBufferBuilder::Release()
* Address remaining allocator-improvements comments
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