insertLen
Inserts @len bytes of @val into @string at @pos.
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.
If @pos is -1, bytes are inserted at the end of the string.
Return
@string
Parameters
pos
position in @string where insertion should happen, or -1 for at the end
val
bytes to insert
len
number of bytes of @val to insert, or -1 for all of @val