linalg: More specific bounds for mat_mul and eye
Array: add .diag_iter_mut()
Array: impl DoubleEndedIterator for one-dimensional elements iterators
Array: Fail doc formatting.
Array: Add indexed versions of the iterators.
Array: specialize fn size() for dimensions 1 ... 3
linalg: Fix transpose symbol
Array: fix clone impls for iterators

We can't use deriving since that inserts a restrictive A: Clone bound.
Array: add .mat_mul_col, matrix multiply with column
linalg: perform cholesky decomposition in-place