Cache constant into_v32 calls

[baehyunsol]

No description provided.

[baehyunsol]

Don't cache everything: cache only frequently used string slices.

Make into_v32 print it's arguments and render the reference MDxt: collect string slices that are used more than 5 times and length is longer than 5!

[baehyunsol]

For gigantic branches like below,

if op == into_v32(...) {...}

else if op == into_v32(...) {...}

else if op == into_v32(...) {...}

// ... hundreds more

use another algorithm (eg: FSM)

[baehyunsol]

Below table shows repetitive into_v32 calls when rendering the reference file (eg: into_v32("math") is called 173 times).

String	Count
"&<>\"'"	2703
"math"	173
"</span></span>\n"	172
"<span class=\"code-fence-row\">"	169
"</span>"	155
"<span class=\"code-fence-code\">"	149
"</code>"	113
"<code class=\"inline-code-span\">"	95
"</td>"	89
"<td>"	88
"\">"	83
"</li>"	83
"</p>"	73
"<p>"	73
"<span class=\"color-white\">"	69
"<li>"	68
"1."	65
"</a>"	58
"<a href=\""	58
"\n"	52
">"	51
"#"	51
" "	48
"\""	48
" id=\""	47
"](#"	46
"- "	46
"["	46
")"	46
"/"	39
"</tr>"	38
"<tr>"	38
""	37
"a"	36
"<h3"	32
"</h3>"	32
"<>*~[|]^`&"	29
"A"	29
"I"	29
"i"	29