MPIX_Continue: the basic implementation

[JiakunYan]

Pull Request Description

The basic implementation of the MPIX continuation proposal (https://github.com/mpiwg-hybrid/hybrid-issues/issues/6).

A brief overview: it introduces a new way to attach a callback function (named continuation) to a request object corresponding to a pending operation and register it with a continuation request. The callback will be invoked once the corresponding operation is complete. The continuation request provides ways to manage the execution and completion of registered continuations.

What this PR is not covered.

• Fortran binding.
• Logic related to attaching callback to continuation requests.
• Logic related to error propagation.