Preparse matrix literals of the form [a,b;c,d]

[robertwb]

E.g.

sage: [1,2;3,4]
[1 2]
[3 4]
sage: [1,2;3,4].parent()
Full MatrixSpace of 2 by 2 dense matrices over Integer Ring
sage: [1, 2; 3, 1/2].parent()
Full MatrixSpace of 2 by 2 dense matrices over Rational Field
sage: [1, 2; 3, pi].parent()
Full MatrixSpace of 2 by 2 dense matrices over Symbolic Ring

[jason]

Of course, a big change like this should at least have a vote and discussion on sage-devel.

Rob Beezer points out the preparser doesn't deal so well with newlines, so this patch breaks this valid python syntax:

sage: ["1","2","\n;"
....: "2","4"]
....: 
....: 
....: 
....: 

and it never quits.

[jason]

See #11699 for another take on using this syntax in the regular matrix constructor.