isl_seq_normalize: use pre-allocated temporary variable in isl_ctx

isl_seq_normalize: use pre-allocated temporary variable in isl_ctx

This requires that we pass along an isl_ctx, but it removes
the need for allocating a temporary variable every time
isl_seq_normalize is called.
In some cases, up to 20% of the execution time of isl_seq_normalize
was wasted on initializing and clearing this temporary variable.