Complete shape inference for splicing, einsum with ellipsis-in-the-middle

From the changelog:

Added

• let%cd _ = (and let%op _ =?) do not affect root tracking (intended for adding shape constraints).
• More expressive shape constraints: allowing row variables to be sandwiched between leftmost axes beg_dims and rightmost axes dims.
• Einsum notation support for leftmost axes.

Changed

• Cleaned up "user-facing" API by moving IDX and CDSL to Train, and Tensor.O to more precise Operation.At.
• Added interface Tensor.mli to reduce "the user learning surface".
• Improved documentation and layout of Shape.mli.
• A more reasonable syntax for labels specifications and einsum notation. In particular, whitespace insensitive (except whitespace not allowed inside identifiers).
• Vendored the npy package while we wait for a PR.

Fixed

• Moved cudajit to depopts.
• Slice shape inference is now complete, by using leftmost axes beg_dims in constraints.