Restore Stephen's expression graph from blaze-attic
Start on overloadable blaze functions and expression graphs
Add some typed ufuncs
Some more work on expression graph
Add some functionality to coretypes
Add initial blaze type unification algorithm
Some more unification and promotion
Handle coercions in unification
Simplify graph nodes to only kernel application and array input (treat scalars as 0d arrays)
Add traits and default coercion rules
Some minor updates thanks to @mwiebe
Merge branch 'expr'