This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Implement the second part of https://bugs.musicpd.org/view.php?id=3990

Enables using range-based "for".

This saves another 3% memory.

Eliminate duplicate code.

Use class TagBuilder instead.

Methods BeginAdd() and EndAdd() have been replaced by class
TagBuilder.

Move to TagSettings.c and use C99 initializers.