Trim the XML output file

[alfsb]

This PR trims the generated Docbook XML file of PHP Manual, assembled by doc-base/configure.php. It strips:

1. The DOCTYPE node (after entity expansion);
2. Duplicated XML namespaces;
3. XML comments;
4. Insignificant whitespaces of Docbook.

Statistics:

• File reduction, bytes: -30,90%, from 44736907 to 30913772;
• File reduction, lines: -68,04%, from 1404683 to 448958;
• Maximum resident set size: -16,31%, from 1781788 to 1491252 kb.
• Time increase: about +0.25s

I'm surprised that the time increase is so small, but more surprised to see a reduction in Maximum resident set size. This means that the peak memory occurs later in configure (or any of its subs scripts), after the initial assembly.

Testing with PhD will be done in future. I expect no functional changes, but a full test is necessary anyways. PhD developers may need to check this very critically. Included a tool to recalculate the list, in the case the RNGs are updated.

Comments and reviews are welcome. Plan to wait at least two weeks before merging.

[alfsb]

Got curious, and tested each reduction.

Final size  Component   Reduction   Red%

44736907	original		
42815943	DTD	        -1920964	-4,29%
39039498	xmlns	    -3776445	-8,44%
33554966	comments	-5484532	-12,26%
30913772	dbwsi	    -2641194	-5,90%

[jordikroon]

I prefer to use this form given it's a static array. But that's preference.

$trimList = [
    // These first five elements account for almost 50%
    // of all insignificant Docbook whitespace in manual.
    'refsect1',
    'varlistentry',
    '...',
];

[alfsb]

I'm delaying the merging of this PR, for now. There are some warnings being silenced on configure.php, and it is easier to check for insignificant differences without a change like this.

[alfsb]

Rebased. Trim doctype, comments and duplicated namespaces. Removed trim of inter element whitespaces for now, as to avoid surfacing new PhD render errors.

Final reductions:

• Time: 11.15 sec to 9.80 sec;
• Peak resident set: 1.80 Gb to 1,50 Gb;
• Lines: 1.45 M to 1.17 M.

Also, exports DOCTYPE node as doc-base/temp/doctype.dtd, so code that needs to know all DTD entities used in manuals can consult this file.

Comments and reviews are welcome. Plan to merge this in one week.