bplist: store high-half u64 that don't overlap s64 as s128

This fixes a long-standing bug where we would serialize unsigned 64-bit
integers >= 0x8000000000000000 in violation of the bplist00 spec. The
spec tells us that _all_ 64-bit values are signed. Those high-half
64-bit integers are meant to be stored as signed 128-bit integers with a
high half of 0x0000000000000000.

Tests have been updated to reflect this.