Reduce reliance on platform-specific APIs, improve Windows support

[MichaelRawson]

Vampire should not need a great deal of platform-specific code. This is a tracking issue to remove as much as possible, and make what we absolutely need as cross-platform as we can.

We currently use platform-specific code:

• [ ] for launching and monitoring processes in portfolio mode
• [x] for monitoring elapsed time
• [ ] for monitoring used instructions
• [x] for semaphores used to synchronise portfolio processes
• [x] for our custom memory allocator
• [ ] because we forgot to remove it

I'm currently looking at whether we can remove the memory allocator and the semaphores. Watch this space!

[MichaelRawson]

Using something akin to UNIX fork() is not really negotiable - we want to keep the existing Vampire state, notably the parsed problem, when we start a strategy - but it doesn't exist on Windows. We will have to rely on Cygwin for this on Windows, and hope that the various incompatibilities don't bite us.

[MichaelRawson]

Allocator is dead, long live Allocator! #469 removed the platform-specific code for allocation.

[MichaelRawson]

No more semaphores, no more semaphore support requirement.

[MichaelRawson]

Replacing global operator new portably to limit memory usage was so painful in the end (#592) that we gave up and call POSIX setrlimit in #594. Ho hum. Better POSIX than Linux.