Cleanup the multithreaded executor

[james7132]

Objective

Improve the code quality of the multithreaded executor.

Solution

• Remove some unused variables.
• Use Mutex::get_mut where applicable instead of locking.
• Use a startup_systems FixedBitset to pre-compute the starting systems instead of building it bit-by-bit on startup.
• Instead of using FixedBitset::clear and FixedBitset::union_with, use FixedBitset::clone_from instead, which does only a single copy and will not allocate if the target bitset has a large enough allocation.
• Replace the Mutex around Conditions with SyncUnsafeCell, and add a Context::try_lock that forces it to be synchronized fetched alongside the executor lock.

This might produce minimal performance gains, but the focus here is on the code quality improvements.

[hymm]

changes are pretty straightforward