Support mixed sparse-dense support when result is sparse

[hameerabbasi]

Lots of changes here, but small ones.

• Added a lot more operators, even if they fail consistently.
• Added support for operators from both sides.
• Moved a lot of functions out below the class.
• (Most important) Where sparse/dense mixed operations are sparse (such as with operator.mul and operator.and_), support that with broadcasting and return a COO array.

[hameerabbasi]

This also supports cases like sparse / dense where (dense != 0).all() holds and sparse ** dense where (dense > 0).all() holds.

[hameerabbasi]

Merging at 18:00 German time if there are no further proposed changes.