Improve error checking in ly:assoc-get and ly:chain-assoc-get.

Improve error checking in ly:assoc-get and ly:chain-assoc-get.

* Introduce a new optional argument strict_checking

* When strict_checking is set to true, output a programming_error
  if the given key is not found in the given alist / achain.

* This patch does not change the current behaviour. It prepares
  a greater modification to remove all assoc calls through
  secure assoc-get calls.

* Remove obsolete chain-assoc-get definition from lily-library.scm