ENH: dispatch(Field, collections.Mapping) for compute_up

[llllllllll]

I also re-exported broadcast_collect through the top-level api, some tests were importing it from there but I cannot understand where it gets there from. These passed on travis but not locally for me so I assume it was coming from and import * on one of the backends

[kwmsmith]

With a little grepping, there are a few other places where we're @dispatching on dict rather than Mapping. Thoughts on converting these as well?

$ ag --python '^@dispatch.*dict'
blaze/compute/core.py
381:@dispatch(Expr, dict)
454:@dispatch(Field, dict)

blaze/compute/python.py
771:@dispatch(Field, dict)

blaze/expr/core.py
368:@dispatch((tuple, list), dict)
373:@dispatch(Node, dict)
386:@dispatch(object, dict)

blaze/expr/expressions.py
342:@dispatch(Symbol, dict)

blaze/interactive.py
185:@dispatch(_Data, dict)

[llllllllll]

I will go over these cases and make sure the conversion makes sense. Also, fun think that I learned while doing this, collections.abc.Mapping does not implement __subclasshook__, so classes must be explicitly registered or actual subclasses. I think it is because the interface is ambigious with a sequence, for example, list implements iter, len, and getitem.

[llllllllll]

I also found some isinstance checks that I think are improved by checking against Mapping

[llllllllll]

ping @kwmsmith tests are passing

[kwmsmith]

LGTM, thanks.