Improved functions index

[Grinnz]

The functions index is directly lifted from perlfunc. This is a good by-category listing but it does not provide a scannable listing with descriptions. It must still be generated from perlfunc from each version so as not to require maintaining our own documentation, but a more useful listing may be possible. The first sentence of the function description may be suitable for most functions but special cases would be needed.

[Grinnz]

Actually perlfunc contains metadata for Pod::Functions that can be directly used for such an index.

[Grinnz]

Pod::Functions only exists in Perl 5.8.0 and newer, and the metadata in perlfunc only exists since 5.16.0.