Add regex-1.5.5 benchmark

[rylev]

This adds the regex-1.5.5 as a benchmark. Here is the comparison in total build times:

• regex:
  • check: 1.83s
  • build: 2.55s
  • build --release: 4.06s
• regex-1.5.5:
  • check: 2.15s
  • build: 3.46s
  • build --release: 7.35s

As you can see, the new version takes a bit longer but still well within the realm of reason.

As for cpu-clock:u (again, I can't measure cycles on my machine):

It appears that the crate takes a bit longer to compile but well within reason and it should definitely exercise the compiler enough to give relevant results.