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>

Conductor was incorrectly saved to Performer tag in MPD database

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

The ones in std have overloads for const char/char.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Fixes breakage of non-ASCII characters, regression from commit
cc72ceb3

Fixes https://github.com/MusicPlayerDaemon/MPD/issues/842

Closes https://github.com/MusicPlayerDaemon/MPD/issues/838

This reduces resource waste for resizing the std::vector in most
cases.

Optimizes a few nanoseconds from the common code path.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/832

Eliminates lots of overhead.

std::all_of becomes constexpr in C++20. I'm not sure it results in better
performance.

Found with useStlAlgorithm
Signed-off-by: Rosen Penev <rosenp@gmail.com>

This consumes more memory (plus 48 kB on 32 bit systems), but reduces
the number of hash collisions, speeding up MPD startup with large
databases.

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

[[maybe_unused]] (introduced in C++17) is standard C++.

https://clang.llvm.org/docs/AttributeReference.html#maybe-unused-unused
says that this is equivalent to the GNU unused attribute.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

The former is deprecated by C++14. The standard says they are the same:

The header defines all types and macros the same as the C standard library
header<stdint.h>.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

The former is deprecated with C++14. The standard says both are the same:

The contents and meaning of the header<cstddef>are the same as the C
standard library header<stddef.h>,except that it does not declare the type
wchar_t, that it also declares the type byte and its associated
operations (21.2.5), and as noted in 21.2.3 and 21.2.4.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

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>

Similar to commits e8f2f980 and
ff1ff1e5

Closes https://github.com/MusicPlayerDaemon/MPD/issues/783

This doesn't work because IterableSplitString() returns its elements
by value.

Fixes clang warning:

 loop variable 'i' is always a copy because the range of type 'IterableSplitString' (aka 'BasicIterableSplitString<char>') does not return a reference [-Werror,-Wrange-loop-analysis]

Found with performance-unnecessary-value-param
Signed-off-by: Rosen Penev <rosenp@gmail.com>

There's just one caller, i.e. ScanGenericTags(), which is documented
to throw exceptions.