Allow Setfield 0.8 (#338)

We get some pretty nice performance improvements when using Setfield 0.8:

![image](https://user-images.githubusercontent.com/11074788/140616974-995d6955-668d-44cb-b81b-2f544d3a6eb8.png)

Ref: TuringLang/AbstractPPL.jl#28

Author: torfjelde

Project.toml

@@ -1,6 +1,6 @@
 name = "DynamicPPL"
 uuid = "366bfd00-2699-11ea-058f-f148b4cae6d8"
-version = "0.16.1"
+version = "0.16.2"
 
 [deps]
 AbstractMCMC = "80f14c24-f653-4e6a-9b94-39d6b0f70001"
@@ -24,6 +24,6 @@ Bijectors = "0.5.2, 0.6, 0.7, 0.8, 0.9"
 ChainRulesCore = "0.9.7, 0.10, 1"
 Distributions = "0.23.8, 0.24, 0.25"
 MacroTools = "0.5.6"
-Setfield = "0.7.1"
+Setfield = "0.7.1, 0.8"
 ZygoteRules = "0.2"
 julia = "1.3"

test/Project.toml

@@ -29,7 +29,7 @@ Documenter = "0.26.1, 0.27"
 ForwardDiff = "0.10.12"
 MCMCChains = "4.0.4, 5"
 MacroTools = "0.5.5"
-Setfield = "0.7.1"
+Setfield = "0.7.1, 0.8"
 StableRNGs = "1"
 Tracker = "0.2.11"
 Zygote = "0.5.4, 0.6"