Enforce validators can only use the correct pubkey type (#2739)

* Enforce validators can only use the correct pubkey type

* adapt to variable renames

* Address comments from #2636

* separate updating and validation logic

* update spec

* Add test case for TestStringSliceEqual, clarify slice copying code

* Address @ebuchman's comments

* Split up testing validator update execution, and its validation

docs/spec/blockchain/state.md

@@ -98,6 +98,10 @@ type Evidence struct {
 type Validator struct {
 	PubKeyTypes []string
 }
+
+type ValidatorParams struct {
+	PubKeyTypes []string
+}
 ```
 
 #### BlockSize