Conductor was incorrectly saved to Performer tag in MPD database

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.

More for https://github.com/MusicPlayerDaemon/MPD/issues/42

Preparing for https://github.com/MusicPlayerDaemon/MPD/issues/42