RPC to query transaction fee + weight + info (#3876)

* initial version for testing

* New version that compiles

* optional at block parameter

* Fix some more view grumbles.

* Update srml/transaction-payment/src/lib.rs

substrate/Cargo.toml

@@ -82,6 +82,7 @@ members = [
 	"srml/aura",
 	"srml/balances",
 	"srml/contracts",
+	"srml/contracts/rpc",
 	"srml/collective",
 	"srml/democracy",
 	"srml/elections",
@@ -109,6 +110,7 @@ members = [
 	"srml/timestamp",
 	"srml/treasury",
 	"srml/transaction-payment",
+	"srml/transaction-payment/rpc",
 	"srml/utility",
 	"node/cli",
 	"node/executor",