timers: clean up for readability

Just doing a little bit of clean up in timers to — hopefully —  make it a bit easier to grok.

• Remove micro-optimizations that no longer yield any benefits
• Simplify Timeout & Immediate to just run all their setup code in the constructor instead of needing extra helper functions
• Simplify Timeout to just have a single argument that determines whether it's repeating or not, instead of having two separate helper functions that differ in only one line
• Adjust timers benchmarks to run long enough to offer meaningful data

I ran a benchmark locally and the performance seemed to remain equivalent but will start a Benchmark CI immediately to confirm.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Affected core subsystem(s)

benchmark, timers