Array: Use Ix and Ixs
linalg: Fix omission in comment
Array: use unicode for M × N in doc.
Add traits for AddGroup, Ring, Field

Num is too restrictive for scalars (Complex doesn't implement it),
so define the basic traits we need.
linalg: Add complex support (optional) to cholesky and least_squares
Array: go over fail cases in docs again, reorder methods.
Array: remove unused bound on FromIterator impl
Array: improve formatting to copy numpy's array repr.
tests: cleanup
Move main.rs into an example file
Array: fix make_unique to only copy the parts that are needed.
Add mat mul bench
Array: update for new itertools Stride

New Stride performs better, which shows in the mat mul benchmark
results.