Implement schema for views of row-tables

[ablaom]

The view of a row table is a table. Row tables implement schema but views do not:

julia> X = Tables.rowtable((x1=rand(5), x2=rand(5)))
5-element Array{NamedTuple{(:x1, :x2),Tuple{Float64,Float64}},1}:
 (x1 = 0.27375482813550467, x2 = 0.7965469426066094) 
 (x1 = 0.11736823853641631, x2 = 0.48949447732525453)
 (x1 = 0.8471399766657528, x2 = 0.48589918772440677) 
 (x1 = 0.5059709917983557, x2 = 0.5492304956299647)  
 (x1 = 0.17045879351929716, x2 = 0.4099462711742099) 

julia> Tables.istable(X)
true

julia> Tables.schema(X)
Tables.Schema:
 :x1  Float64
 :x2  Float64

julia> Xv = view(X, 1:2)
2-element view(::Array{NamedTuple{(:x1, :x2),Tuple{Float64,Float64}},1}, 1:2) with eltype NamedTuple{(:x1, :x2),Tuple{Float64,Float64}}:
 (x1 = 0.27375482813550467, x2 = 0.7965469426066094) 
 (x1 = 0.11736823853641631, x2 = 0.48949447732525453)

julia> Tables.istable(Xv)
true

julia> Tables.schema(Xv) === nothing
true

[quinnj]

Implemented in #131