Optimize string slicing.

[soasme]

As revealed in #48, since() is the most frequently called proc. This PR is aimed to eliminate all since() calls.

Comparing to the current master, it provides roughly 40% performance improvement when running against out2.txt, a 100k doc.

Before:

$ nim c -d:release src/markdown.nim && time ./src/markdown < /tmp/out2.txt
real	0m1.751s
user	0m0.946s
sys	0m0.013s

After:

real	0m1.663s
user	0m0.535s
sys	0m0.016s