Use view for RNN gate slice extraction

This was originally passed over in #907, but I don't find that argument particularly compelling as the return value is only ever used once. Any negative impact on caching is going to happen anyhow during the slice materialization, so we might as well just let the subsequent fused broadcasts handle said materialization for us while reducing allocations.

src/layers/recurrent.jl

@@ -1,7 +1,7 @@
 
 gate(h, n) = (1:h) .+ h*(n-1)
 gate(x::AbstractVector, h, n) = @view x[gate(h,n)]
-gate(x::AbstractMatrix, h, n) = x[gate(h,n),:]
+gate(x::AbstractMatrix, h, n) = view(x, gate(h,n), :)
 
 # Stateful recurrence
 
@@ -97,7 +97,7 @@ struct RNNCell{F,A,V,S}
   state0::S
 end
 
-RNNCell(in::Integer, out::Integer, σ=tanh; init=Flux.glorot_uniform, initb=zeros32, init_state=zeros32) = 
+RNNCell(in::Integer, out::Integer, σ=tanh; init=Flux.glorot_uniform, initb=zeros32, init_state=zeros32) =
   RNNCell(σ, init(out, in), init(out, out), initb(out), init_state(out,1))
 
 function (m::RNNCell{F,A,V,<:AbstractMatrix{T}})(h, x::Union{AbstractVecOrMat{T},OneHotArray}) where {F,A,V,T}
@@ -194,7 +194,7 @@ function Base.getproperty(m::LSTMCell, sym::Symbol)
   elseif sym === :c
     Zygote.ignore() do
       @warn "LSTMCell field :c has been deprecated. Use m::LSTMCell.state0[2] instead."
-    end  
+    end
     return getfield(m, :state0)[2]
   else
     return getfield(m, sym)
@@ -273,7 +273,7 @@ struct GRUv3Cell{A,V,S}
 end
 
 GRUv3Cell(in, out; init = glorot_uniform, initb = zeros32, init_state = zeros32) =
-  GRUv3Cell(init(out * 3, in), init(out * 2, out), initb(out * 3), 
+  GRUv3Cell(init(out * 3, in), init(out * 2, out), initb(out * 3),
             init(out, out), init_state(out,1))
 
 function (m::GRUv3Cell{A,V,<:AbstractMatrix{T}})(h, x::Union{AbstractVecOrMat{T},OneHotArray}) where {A,V,T}