Simpler FIFOLock

[kuszmaul]

This PR provides SimpleFIFOLock, effectively another implementation of FIFOLock.

It's about 1/3 less code, and is simpler: It just uses simple condition-variable programming with no protocol on atomic variables.

On Xeon(R) Platinum 8488C for the following benchmark:

@btime begin lock($f); unlock($f); end

I see the following performance:

13ns for SpinLock
17ns for ReentrantLock
33ns for FIFOLock
35ns for SimpleFIFOLock

So it is a little slower than FIFOLock, but maybe the extra simplicity makes it preferable.

[kuszmaul]

I'm not proposing to actually merge this PR. For one thing, it would seem strange to have two implementations of the same thing (FIFOLock and SimpleFIFOLock).