Extend support for  matrix[[i,j]] syntax

With the GAP branch sl/list-extensions, this now allows the following
operations with singular matrices, including (big)intmats:
* mat[[i,j]] := val;
* IsBound(mat[[i,j]]);
* Unbind(mat[[i,j]]);  # always fails / errors out
Various minor tweaks, cleanups, code formatting, ...
Makefile.am: use -R in SingularInterface_la_LDFLAGS

This ensures our kernel extension will work even if the Singular .la
files are not installed.
Version 2014.09.23