rewrite rules

It is often useful to think of some expressions in terms of other expressions. [dask](http://dask.pydata.org/en/latest/optimize.html?#rewrite-rules) already has an API for expressing a rule set for rewriting some expressions in terms of others so we should look into making them blaze support this too.

Some use cases that we have identified (not complete)
- lean projections (move selection of columns up as early as possible to)
- backend specific optimizations (`a + a + ... a -> a * n`)
- reduce code per backend by implementing exprs in terms of more primitive expressions (head / tail in terms of slice)

Start with the linked Dask rewrite-rules API and compare its approach with Blaze's expression and backend concepts. Use the listed projection, backend-specific optimization, and primitive-expression use cases to define the scope. Done means an agreed rewrite-rule design that supports these cases.

Written by the indexing model from the issue text.