doc: update dependencies (+Storable, Data::Dumper)

The new IPC stuff doesn't work without Storable or Sereal.
Storable is part of the standard library since Perl 5.8, so
we'll put a hard dependency on it for distros that package
it separately.

Data::Dumper is also part of the standard library, and
PublicInbox::MboxReader uses it, and it's frequently useful
during development.

We'll also trim down INSTALL for standard library modules so
it's hopefully less daunting for new users.

Development dependencies are noted in HACKING, now.

Email::MIME is only used for maintainer tests, so it's only
documented in HACKING.

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