Improve performance by a lot (although quite a dirty patch)

[ghost]

Right now nim-markdown recompiles all regular expressions on EACH call to any method/proc, so that if some proc is called 100 times, re module (which uses pcre) has to recompile the same regular expression 100 times - see documentation of https://nim-lang.org/docs/re.html#re%2Cstring

In the long run it would be better to actually define variables with precompiled expressions like:

let someNameForMyRe = re"abc"

proc test = 
  let a = match("helloabc", someNameForMyRe)

but this patch will work for now.

For a simple MD document (taken from Casa) - https://gist.github.com/Yardanico/3ed52233c29b6328bedce30a9f332c3a

I got a ~8x time improvement - from 27.5ms to 3ms:

# Without patch
name ............................... min time      avg time    std dv   runs
test .............................. 26.906 ms     27.842 ms    ±0.512   x179
# With patch
name ............................... min time      avg time    std dv   runs
test ............................... 2.936 ms      3.022 ms    ±0.086  x1000

This is already much better, but I think that we can make it even faster :)

[ghost]

Another benchmark (with https://github.com/treeform/benchy):

import benchy
import markdown

let data = "# Hello world"

timeIt "test":
  let a = markdown(data)
  keep(a)

With the patch:

name ............................... min time      avg time    std dv   runs
test ............................... 0.019 ms      0.024 ms    ±0.004  x1000

Without it:

name ............................... min time      avg time    std dv   runs
test ............................... 0.326 ms      0.345 ms    ±0.007  x1000

14x time improvement for a markdown hello-world (from 0.35ms to 0.025ms)!

[ghost]

By the way, I checked and all tests pass just fine.

[ghost]

The {.threadvar.} annotation makes this caching thread-safe (each different thread will have to compile regular expressions by itself, but I think that this is fine)

[ghost]

Related: #48

[soasme]

Awesome. I like the solution!