Removed compat support for Pike 7.4.
IOBuffer: Use add_ref().
Added read_sint for read signed integer.

The reverse might be more obvious (read_int for signed, read_uint for
unsigned), but at least it exists now.

Also fixed so that add_int works for all negative numbers, and
optimized it rather significantly for bignums (it no longer creates a
string copy of the bignum value).

Also added a simple add_padding() function to add a few bytes of data
set to a specific value (basically memset)