Move optimizer into core

This only moves the files, adjusts the build system, exports APIs
and does minor fixups to make sure the code builds.

This does not yet try to make the

Move optimizer into core

This only moves the files, adjusts the build system, exports APIs
and does minor fixups to make sure the code builds.

This does not yet try to make the optimizer usable independently
of opcache.

Closes GH-6642.

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