Stack allocation

Class protected_fixedsize
Class pooled_fixedsize_stack
Class fixedsize_stack
Class segmented_stack
Class stack_traits
Class stack_context
Support for valgrind
Support for sanitizers

The memory used by the stack is allocated/deallocated via a StackAllocator which is required to model a stack-allocator concept.

A StackAllocator must satisfy the stack-allocator concept requirements shown in the following table, in which a is an object of a StackAllocator type, sctx is a stack_context, and size is a std::size_t:

expression	return type	notes
`a(size)`		creates a stack allocator
`a.allocate()`	`stack_context`	creates a stack
`a.deallocate( sctx)`	`void`	deallocates the stack created by `a.allocate()`

	Important
	The implementation of `allocate()` might include logic to protect against exceeding the context's available stack size rather than leaving it as undefined behaviour.

	Important
	Calling `deallocate()` with a `stack_context` not set by `allocate()` results in undefined behaviour.

	Note
	Depending on the architecture `allocate()` stores an address from the top of the stack (growing downwards) or the bottom of the stack (growing upwards).