Get rid of some quadratic behavior

[umanwizard]

markdown-match-bold and markdown-match-italic both call markdown-inline-code-at-pos-p on each candidate match, which takes linear time in the distance
from the start of the block to the point being checked.

In a large block with many candidate matches inside inline blocks, this is slow because the function is called with each candidate, resulting in overall quadratic behavior, which this commit fixes by starting the code-at-pos scan after the last already-found match.

For example, the following benchmark code:

(with-temp-buffer
  (dotimes (_ 400)
    (insert "`my_test_string`\n"))
  (markdown-mode)
  (car (benchmark-run
           (font-lock-debug-fontify))))

takes about 10.5 seconds before this commit, and about 0.14 seconds after this commit.

Description

Related Issue

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• Improvement (non-breaking change which improves an existing feature)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I have read the CONTRIBUTING.md document.
• I have updated the documentation in the README.md file if necessary.
• I have added an entry to CHANGES.md.
• I have added tests to cover my changes.
• All new and existing tests passed (using make test).
  (except test-markdown-export/buffer-local-css-path which was already failing)

[umanwizard]

This fixes at least some of the issues contributing to #799 and #391, although I think there is more slow behavior lurking, so I won't claim those issues are entirely fixed.

[hmelman]

Thanks, this does fix a lot of #391 for me.