Codechange: Replace custom mutex code with C++11 mutex'es.

A conforming compiler with a valid <mutex>-header is expected.
Most parts of the code assume that locking a mutex will never fail unexpectedly,
which is generally true on all common platforms that don't just pretend to
be C++11. The use of condition variables in driver code is checked.

Codechange: [core] Implement SmallVector using std::vector

The public and protected interface to SmallVector are unchanged
SmallVector now requires that items be default constructible
This isn't an issue since some contained items were previously created
uninitialized.

Temporary default constructors are added to the following structs
- SmallPair
- SmallStackItem
- GRFPresence

Where vector<bool> is required, transition immediately to std::vector
to avoid returning proxy object references.