Fix unsafe and inefficient uses of copy_string

Add a new overload to `copy_string` that takes the input characters plus
size. The new overload avoids inefficient scanning of the input for the
null terminator in cases where we know the size beforehand. Furthermore,
this overload *must* be used when dealing with input character sequences
that may have no null terminator, e.g., when the input is from a
`std::string_view` object.

src/Val.h

@@ -546,6 +546,7 @@ public:
     int Len() const;
     const u_char* Bytes() const;
     const char* CheckString() const;
+    std::pair<const char*, size_t> CheckStringWithSize() const;
 
     // Note that one needs to de-allocate the return value of
     // ExpandedString() to avoid a memory leak.