Reduce compiler warnings in rlp (#674)

eth/rlp.nim

@@ -393,7 +393,7 @@ proc readImpl(rlp: var Rlp, T: type[object|tuple],
   template getUnderlyingType[T](_: Option[T]): untyped = T
   template getUnderlyingType[T](_: Opt[T]): untyped = T
 
-  template op(RecordType, fieldName, field) =
+  template op(RecordType, fieldName, field) {.used.} =
     type FieldType {.used.} = type field
     when hasCustomPragmaFixed(RecordType, fieldName, rlpCustomSerialization):
       field = rlp.read(result, FieldType)

eth/rlp/object_serialization.nim

@@ -11,7 +11,7 @@ template rlpCustomSerialization* {.pragma.}
   ## a reference to the object holding the field.
 
 template enumerateRlpFields*[T](x: T, op: untyped) =
-  type RecordType = type x
+  type RecordType {.used.} = type x
   for fieldName, field in fieldPairs(x):
     when not hasCustomPragmaFixed(RecordType, fieldName, rlpIgnore):
       op(RecordType, fieldName, field)
@@ -21,7 +21,7 @@ proc rlpFieldsCount*(T: type): int =
 
   proc helper: int =
     var dummy: T
-    template countFields(RT, n, x) = inc result
+    template countFields(RT, n, x) {.used.} = inc result
     enumerateRlpFields(dummy, countFields)
 
   const res = helper()

eth/rlp/writer.nim

@@ -173,7 +173,7 @@ proc hasOptionalFields(T: type): bool =
   proc helper: bool =
     var dummy: T
     result = false
-    template detectOptionalField(RT, n, x) =
+    template detectOptionalField(RT, n, x) {.used.} =
       when x is Option or x is Opt:
         return true
     enumerateRlpFields(dummy, detectOptionalField)
@@ -286,7 +286,7 @@ proc appendRecordType*(self: var RlpWriter, obj: object|tuple, wrapInList = wrap
     else:
       self.startList(fieldsCount)
 
-  template op(RecordType, fieldName, field) =
+  template op(RecordType, fieldName, field) {.used.} =
     when hasCustomPragmaFixed(RecordType, fieldName, rlpCustomSerialization):
       append(self, obj, field)
     elif (field is Option or field is Opt) and hasOptional: