Fix Bugs

William

• http://trac.sagemath.org/sage_trac/ticket/12689 -- (done) Segfault in solve_left for sparse matrices over ZZ

• http://trac.sagemath.org/sage_trac/ticket/11358 -- (done) matrix multiplication over ZZ sometimes gives incorrect results

• http://trac.sagemath.org/sage_trac/ticket/10281 -- (needs review) Multimodular echelon form over cyclotomic fields fails