Stack allocation

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## Stack allocation

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

#### stack-allocator concept

A stack-allocator must satisfy the stack-allocator concept requirements shown in the following table, in which `a` is an object of a stack-allocator 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
The stack is not required to be aligned; alignment takes place inside execution_context.

| 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).