string_bytes: add StringBytes::IsValidString()

Performs a quick, non-exhaustive check on the input string to see if
it's compatible with the specified string encoding.

Curently it only checks that hex strings have a length that is a
multiple of two.

src/string_bytes.h

@@ -36,6 +36,11 @@ using v8::Value;
 
 class StringBytes {
  public:
+  // Does the string match the encoding? Quick but non-exhaustive.
+  // Example: a HEX string must have a length that's a multiple of two.
+  // FIXME(bnoordhuis) IsMaybeValidString()? Naming things is hard...
+  static bool IsValidString(Handle<String> string, enum encoding enc);
+
   // Fast, but can be 2 bytes oversized for Base64, and
   // as much as triple UTF-8 strings <= 65536 chars in length
   static size_t StorageSize(Handle<Value> val, enum encoding enc);