More greppin speedup trickery

 I learned about SIMD based hyperscan regex scanning libs being

super fast, so I refactored grab a bit to make it possible

to load different regex engines at runtime for speed comparison.

I was also told about a quite popular similar project and

compared it to my greppin branch. Enjoy!

Still need to check whether and how it would be possible to

vectorize the matching on files to fully exploit SIMD. Will

keep you updated!

Update:I checked the code of hs_scan_vector() and it's just

iterating over the scatter array and calling internal scan

functions on it. I thought it could be using SIMD for it too,

but I was stupid. So, no more speedup on that front.

While digging into that topic, I noticed that apparently quite

lot of NIDS technology is still relying on regexes in 2020 (lol).