initialization of matrices from vectors or list of lists can be way faster

[mderickx]

This is demonstrated by the code below:

sage: M=MatrixSpace(GF(46337),400)
sage: m=M.random_element()
sage: m1=m.columns()
sage: time M(m1)
CPU times: user 3.06 s, sys: 0.44 s, total: 3.49 s
Wall time: 3.70 s
400 x 400 dense matrix over Finite Field of size 46337
sage: x=[]
sage: time map(x.extend,m1)
CPU times: user 0.81 s, sys: 0.02 s, total: 0.83 s
sage: time M(x)
CPU times: user 0.01 s, sys: 0.00 s, total: 0.01 s
Wall time: 0.01 s
400 x 400 dense matrix over Finite Field of size 46337
sage: M(x)==M(m1)
True