The former was deprecated with C++14.

According to the C++11 and C++17 standards, both files are identical.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Found with modernize-use-equals-default
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Prepare for allowing ISO8601 time stamps without delimiters, such as
20191216, and prevent them from being interpreted as numeric UNIX time
stamps.

Closes #410

This feature requires `libpcre`.

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.

Closes #397

Prepare to allow more complex expressions.

Required for "modified-since".

All DetachedSong instances should be converted to LightSong first,
which is a very cheap operation.

Closes #89

Will allow more complex fitler expression, such as negation (#89).