song/Filter: operator "==" never searches substrings in filter expressions

The protocol documentation says that the difference between `find` and
`search` is that `search` is case insensitive, but that's only half
the truth: `search` also searches for sub strings instead of matching
the whole string.  This part is undocumented and unfortunate, but at
this point, we can't change it.

However leaking this surprising behavior to the new filter expressions
was a bad idea; the "==" operator should never match substrings.  For
people who need that, we should add a new operator.

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