Tweak the initial pool in Perl bindings, so that perl -MSVN::Core -c -e1 won't segfault.
* perl/native/Core.pm: (gpool): New function for returning and holding the global pool. This is to avoid 'our's which appears to have different destruction order. (SVN::Pool::DESTROY): Skip destroy for SCALAR-typed SVN::Pool, as they appear to be caused by bugs in Perl.
* perl/native/Ra.pm: Use the gpool method to get the pool for init_ra_libs.
* Core.pm, Fs.pm, Ra.pm: Add more documentation for some parts of the Perl API. No changes to the actual code, except for some tidying of whitespace. There are 'TODO' markers in some of this, but for the most part they act as warnings of unexpected behaviour and unimplemented features, so it's probably best to leave them in.