ENH: add `mdot`: fast dot with multiple arguments.

This is not ready to be merged!

`mdot` makes it easy to chain `dot`. Think of `mdot` as this:

    def mdot(*args): return reduce(np.dot, args)

However, `mdot` uses the fact that matrix multiplication is associative and
finds the optimal order of multiplications with dynamic programing before
doing the mutiplication. This can speed up the multiplication quite a bit.

The algorithm follows:
Cormen, "Introduction to Algorithms", Chapter 15.2, p. 370-378

Note: currently `mdot` only works with arrays which have `ndim == 2` to avoid
ambiguities of `dot`. Also see the discussion in #4311.

TODO:
- [x] Decide if `ndim != 2` should be ok
- [x] Add unittests
- [x] decide on name. possible names are:
  - `mdot`: think "multi dot"
  - `multidot`: as above :)
  - `multi_dot`: as above :)
  - `chain_dot`
  - `fdot`: think "fast dot" (should the parameter optimize be optional then? I don't think so.)
  - `ddot`: think "multiple d for multiple dot"
  - `cdot`: think "chain dot"
  - `odot`: think "optimal do

Maybe say something like "optimally ordered matrix product" rather than "fast dot product"? That would be more obvious for readers (such as myself) who are already vaguely aware that such an algorithm exists.