String.prependLen

Prepends len bytes of val to string.

If len is positive, val may contain embedded nuls and need not be nul-terminated. It is the caller's responsibility to ensure that val has at least len addressable bytes.

If len is negative, val must be nul-terminated and len is considered to request the entire string length. This makes glib.string_.String.prependLen equivalent to glib.string_.String.prepend.