refactor: Simplify string filters to use stringify

Sources/Filters.swift

@@ -1,35 +1,13 @@
-func toString(_ value: Any?) -> String? {
-  if let value = value as? String {
-    return value
-  } else if let value = value as? CustomStringConvertible {
-    return value.description
-  }
-
-  return nil
-}
-
 func capitalise(_ value: Any?) -> Any? {
-  if let value = toString(value) {
-    return value.capitalized
-  }
-
-  return value
+  return stringify(value).capitalized
 }
 
 func uppercase(_ value: Any?) -> Any? {
-  if let value = toString(value) {
-    return value.uppercased()
-  }
-
-  return value
+  return stringify(value).uppercased()
 }
 
 func lowercase(_ value: Any?) -> Any? {
-  if let value = toString(value) {
-    return value.lowercased()
-  }
-
-  return value
+  return stringify(value).lowercased()
 }
 
 func defaultFilter(value: Any?, arguments: [Any?]) -> Any? {