makeleaf passes on symbols
style edits
Small typo
Minimal test case that fails on master
Merge pull request #891 from cpcloud/mrocklin-compute-core-fixes

Compute core improvements
Implement first/last reductions
Make first and last functions
xfail on last because negative slicing is not well supported in python
No need for SingleElementSlice anymore
Break out reductions to a single function
Implement slice for pytables Column
Add first and last to reduction test
Allow non reduction non collection expressions
Failing reduction on slices
Change reductions to use schema instead of _dtype
Only Reductions for now
put b_u_u_t_b back to the way it was
lf -> leaf
Add reduction usage test for first/last
Assume we have toolz latest