refactors GenerateOggSerial into a generic GenerateSerial
utility, under the util lib.

libFLAC may be encoded without Ogg support. If Ogg support is disabled,
libFLAC will still export Ogg-related methods (like setting a serial
number), and throw a runtime error when initializing an Ogg stream.

GenerateOggSerial does not depend on libogg. Refactoring it into
a generic GenerateSerial prevents having to add build-time checks
for libogg within the FLAC encoder plugin.

This commit adds some tags that are (mostly) interesting for listeners
of classical music.

Ensemble
--------
This is an ensemble that is playing the music, such as Wiener
Philharmoniker. The tag can be used to distinguish the ensemble from the
conductor, composer, soloist, and ensemble, that are generally all in
the "ARTIST" tag.

Movement
-------
The movement number and movement (name) of this track, i.e.  "II" and
"Allegro".

ComposerSort
------------
Allows us to look for Beethoven's 9th under B, for Beethoven, not L for
Ludwig.

Location
--------
This is the location of the recording, e.g. "Wiener Musikverein".

Shorter.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Replaces the static Duplicate() method.

To make things simpler, AllocatedString is now a non-template class.

Very rough draft.  Barely works.

Preparing for a variant of TimerEvent with coarse 1-second
granularity, but cheaper (with a timer wheel).

Replaces the static Duplicate() method.

To make things simpler, AllocatedString is now a non-template class.

This eliminates some complexity from class CurlRequest.

Hooray C++17!

This template was never instantiated, so the problem never occurred.