Swig_offset_string moved to misc.c
Minor tweaks in Swig_feature_set
Fix template partial specialization detection

Fixes template definitions that should be ignored and warnings introduced after nested changes:
../../../Examples/test-suite/refcount.i:63: Warning 318: Instantiation of template 'RCPtr< A >' is ambiguous,
../../../Examples/test-suite/refcount.h:159: Warning 318: instantiation 'RCPtr< T >::RCPtr' used,
../../../Examples/test-suite/refcount.h:159: Warning 318: instantiation 'RCPtr< T >::RCPtr' ignored.
../../../Examples/test-suite/refcount.h:159: Warning 318: instantiation 'RCPtr< T >::RCPtr' ignored.
../../../Examples/test-suite/refcount.h:159: Warning 318: instantiation 'RCPtr< T >::RCPtr' ignored.

Different/clearer implementation of constructor detection since Classprefix is
not behaving the same since nested changes were introduced. Only accept
constructors if in extend mode or in a class.

Also remove unused "isextension" attribute on the nodes.