timing: Move towards DelayPairs for timing analysis

[rowanG077]

My plan is to first implement hold time violations report. This will happen in the next PR. This PR is only moving more things in the timing engine and the reporting to DelayPair. Once that is done I can use the DelayPairs to implement a hold time violation report.

After the hold time reports I will add false path min/max delay using the same mechanism.

TODO:

• Run performance difference on realistic design.
• Verify same timing output

[rowanG077]

Ran a 20K lut design through it 5 times. There seems to be no runtime change or it is drowned out by measurement error.

Timing information reported was the same. I compared the detailed net timings in two json reports using the same seed. This PR does change the json report slightly as now it outputs both the min and max delay.