#13 standard library, pre-load Scheme libraries

[Oleg Paraschenko]

It makes sense to create a library of re-usable Scheme
functions, such as xlib:escape-for-c. Load them is the
environment variable XSIEVE_PRELOAD_FILES (separated by
":" or by ";".

Go through examples (such as xslt2xsieve) and move
Scheme code to libraries.

[Oleg Paraschenko]

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Fix the documentation for xslt2xsieve example. ("The
function <literal>xlib:escape-for-string</literal> is
defined in the file <filename>xslt2xsieve.scm</filename>.")