Improve performance about commonmark-hs

[jgm]

What I've tried

• rewriting to operate directly on Text instead of tokenizing first
• rewriting to operate directly on Text, using megaparsec instead of parsec, and using the fast parsers takeWhileP etc.
• rewriting to use ByteStrings instead of Texts in the Toks.

None of this achieved any speed improvement over the current version using [Tok]; indeed, in every case performance was worse.

Profiling reveals that block structure parsing is fast. Most of the time is taken up by tokenize and restOfLine (31%), and by inline parsing.

Instructions for profiling

make prof

Current results (March 12 2020):

1.8 	 parseChunks
2.1 	 pDelimChunk
2.2 	 Commonmark.Blocks.runInlineParser
2.5 	 blockContinues
2.6 	 Commonmark.Inlines.processBs
2.9 	 MAIN
3.9 	 block_starts
6.6 	 renderHtml
9.0 	 pSymbol
11.9 	 defaultInlineParser
17.5 	 Commonmark.Tokens.tokenize
32.6 	 restOfLine

[jgm]

For a 1.4MB file:

[jgm]

Benchmarks for different extensions:

extension	mean
-xautolinks	310.8 ms (309.3 ms .. 311.3 ms)
-xpipe_tables	295.2 ms (293.2 ms .. 296.6 ms)
-xstrikethrough	267.9 ms (265.6 ms .. 269.1 ms)
-xsuperscript	267.8 ms (264.9 ms .. 269.5 ms)
-xsubscript	266.8 ms (263.6 ms .. 267.9 ms)
-xsmart	293.0 ms (292.0 ms .. 294.3 ms)
-xmath	287.4 ms (285.4 ms .. 290.7 ms)
-xemoji	281.6 ms (280.3 ms .. 282.8 ms)
-xfootnotes	291.3 ms (286.1 ms .. 293.3 ms)
-xdefinition_lists	272.6 ms (271.0 ms .. 275.4 ms)
-xfancy_lists	271.2 ms (269.3 ms .. 273.8 ms)
-xattributes	284.2 ms (283.4 ms .. 285.7 ms)
-xraw_attribute	280.7 ms (279.6 ms .. 281.6 ms)
-xbracketed_spans	268.5 ms (267.0 ms .. 269.4 ms)
-xfenced_divs	269.6 ms (267.5 ms .. 271.6 ms)
-xauto_identifiers	274.9 ms (273.0 ms .. 277.8 ms)
-ximplicit_heading_references	269.8 ms (268.2 ms .. 272.8 ms)
-xall	520.4 ms (515.5 ms .. 523.6 ms)

[jgm]

One idea to explore: use ShortText from text-short package instead of Text in Tok.
The public API could still use Text.
This should reduce the memory used by the tokens.