Require PPI-1.218...

which now has the non-compatible changes that were
in 1.216_01 but are now in a production release.
Upgrade to Readonly-2.00

Readonly-2.00 obviates the need for Readonly::XS.
In fact, they now conflict with each other. Fixes #584
Remove reference to deprecated RcsKeyword policy.
Remove workaround for Readonly::XS loading.

Now that Readonly::XS is dead and not an
optional dependency, this workaround is
irrelevant.
Fixe self-compliance.
Bump VERSION to 1.122
Fix POD syntax error.
Fix self-compliance.
Update change log.
Fix POD and spelling errors.
All recommended/optional dependencies are now required.

This ensures a consistent & optimal experience for all users.
Also, it makes our code a lot simpler since we don't have to
generate wrappers that run all the tests without the optional
modules.

Generally speaking, I feel we should avoid recommended/optional
dependencies unless there is a really compelling reason to
permit users to not have them (like because they don't work
on certain platforms or perls).
Regen MANIFEST.
Fixed a typo in the Variables::ProhibitPerl4PackageNames message.
Makefile.PL is no longer supported
handle '=' and '!=' in is_assignment_operator