OpenSystemsDevelopment / qpdf

Refactor QPDF_Stream to use stream filter classes to handle supported
stream filters as well.

Fixes qpdf/qpdf#417

Thanks to Zdenek Dohnal <zdohnal@redhat.com> for contributing the code
used for the gnutls crypto provider.

ClosedFileInputSource is an input source that keeps the file closed
when not reading it.

This is the beginning of higher-level API support using helper
classes. The goal is to be able to add more helpers without continuing
to pollute QPDF's and QPDFObjectHandle's public interfaces.

Implement a TokenFilter class and refactor Pl_QPDFTokenizer to use a
TokenFilter class called ContentNormalizer. Pl_QPDFTokenizer is now a
general filter that passes data through a TokenFilter.

Additional testing is added in later commits to be supported by
additional changes in the library.

Rather than checking consistency of libqpdf.map, generate it.

The build now checks to make sure libqpdf.map has the right library
version number in it.

Also update maintainer documentation on binary compatibility testing.

Add new RandomDataProvider object and implement existing random number
generation in terms of that.  This enables end users to supply their
own random data providers.