Define getindex on rules (#57)

We define `iterate` on them for convenience, and `getindex` provides a
similar convenience for cases where you're not sure whether the rules
resulting from a call to `rrule`/`frule` will be a `Tuple`. So instead
of writing `partials isa Tuple ? partials[i] : partials`, you can now
just write `partials[i]`.

src/rules.jl

@@ -54,6 +54,10 @@ abstract type AbstractRule end
 Base.iterate(rule::AbstractRule) = (rule, nothing)
 Base.iterate(::AbstractRule, ::Any) = nothing
 
+# This ensures we don't need to check whether the result of `rrule`/`frule` is a tuple
+# in order to get the `i`th rule (assuming it's 1)
+Base.getindex(rule::AbstractRule, i::Integer) = i == 1 ? rule : throw(BoundsError())
+
 """
     accumulate(Δ, rule::AbstractRule, args...)
 

test/rules.jl

@@ -12,14 +12,16 @@ cool(x) = x + 1
         @test rrx == 2
         @test rr(1) == 1
     end
-    @testset "iterating rules" begin
+    @testset "iterating and indexing rules" begin
         _, rule = frule(+, 1)
         i = 0
         for r in rule
             @test r === rule
             i += 1
         end
         @test i == 1  # rules only iterate once, yielding themselves
+        @test rule[1] == rule
+        @test_throws BoundsError rule[2]
     end
     @testset "helper functions" begin
         # Hits fallback, since we can't update `Diagonal`s in place