~4x performance improvement on spec.txt by removing autolink_uri Regex
~4x performance improvement on spec.txt by removing autolink_uri Regex

scan_autolink_uri replaced with a custom implementation that does not
use Regular Expressions. By running against spec.txt the new average
time is 58-78ms vs ~250ms before the change.

lostfictions/CommonMark.NET,dudb/CommonMark.NET,dudb/CommonMark.NET,Mosoc/CommonMark.NET,dut3062796s/CommonMark.NET,lostfictions/CommonMark.NET,laedit/CommonMark.NET,dut3062796s/CommonMark.NET,AMDL/CommonMark.NET,laedit/CommonMark.NET,AMDL/CommonMark.NET,Mosoc/CommonMark.NET