Inserts @len bytes of @val into @string at @pos.
Because @len is provided, @val may contain embedded
nuls and need not be nul-terminated. If @pos is -1,
bytes are inserted at the end of the string.
Since this function does not stop at nul bytes, it is
the caller's responsibility to ensure that @val has at
least @len addressable bytes.
Inserts @len bytes of @val into @string at @pos. Because @len is provided, @val may contain embedded nuls and need not be nul-terminated. If @pos is -1, bytes are inserted at the end of the string.
Since this function does not stop at nul bytes, it is the caller's responsibility to ensure that @val has at least @len addressable bytes.