[Parser] Remove an unnecessary use of mutating

Author: hamishknight

Sources/SwiftParser/TokenConsumer.swift

@@ -138,7 +138,7 @@ extension TokenConsumer {
   /// If this is the case, return the `Subset` case that the parser is positioned in
   /// as well as a handle to consume that token.
   @inline(__always)
-  mutating func at<SpecSet: TokenSpecSet>(
+  func at<SpecSet: TokenSpecSet>(
     anyIn specSet: SpecSet.Type
   ) -> (spec: SpecSet, handle: TokenConsumptionHandle)? {
     #if SWIFTPARSER_ENABLE_ALTERNATE_TOKEN_INTROSPECTION