* Cbits archive for c, cxx, cmm, asm sources

Build and install extra archive file for non-Haskell code.
It allows to build alternative backends and compilation pipelines for GHC.
GHC plugins can observe and process Haskell related code,
while the 'cbits' archive supply the foreign code parts.

* Install extra module related (modpak) files when available

Support '.modpak' file installation. Modpak is an optional file,
it is generated by GHC plugins (or other tools) for Haskell modules.
The modpak file format is not set, it could be a zip file or other format.
In modpak files GHC plugins can store additional data for Haskell modules.
With modpak files Cabal can support alternative GHC backends out of the box.

* add changelog entry for cbits and modpak feature

* Support for extra compilation artifacts

GHC plugins can store custom data in the 'extra-compilation-artifacts' directory
which gets installed with the package.

This is a generalization of the previous design. See the discussion in PR #8662.

* documentation for extra-compilation-artifacts installation