Unify Reals,Bint into Array

[fritzo]

Addresses #322, #351 , #348

This unifies BintType and RealsType types into a common ArrayType with support for .dtype and .shape (I have unmarked those deprecated). After this PR Bint[size, i,j,k] denotes a homogeneously-bounded i x j x k-sized array of Bint. This PR does not yet use those integer valued arrays in distributions or elsewhere; that is for follow-up.

This PR also clarifies the role of Bint. Whereas I had previously thought we would need heterogeneously-bounded distributions (as e.g. requested of dist.Multinomial), I now believe we can restrict to homogeneously-bounded Bint types, and add a separate heterogeneous type Tuple[Bint[2], Bint[3]) for indexing into arrays. This PR does not add that Tuple, but it does clarify the planned type hierarchy:

Domain
+-- ArrayType                                 \
|   +-- RealsType                              }  support .dtype, .shape
|   +-- BintType  # homogeneously bounded     /
+--TupleType      # heterogeneously bounded   }   does not support .dtype, .shape

Tested

• pure refactoring is covered by existing tests

[eb8680]

Looks good per our discussion earlier