When not direct mapped, make index constructors private and add factory
functions for channel and index.
Co-authored-by: GabrielDeml <gabrielddeml@gmail.com>
This makes callback registration completely thread safe.
This patch also uses templates and macros to dramatically reduce the amount of
manual boilerplate.
The old headers were moved into folders because doing so avoids polluting
the system include directories.
Folder names were also normalized to lowercase.
