Auto merge of #119302 - Mark-Simulacrum:relative-spans, r=WaffleLapkin

Support encoding spans with relative offsets

The relative offset is often smaller than the absolute offset, and with
the LEB128 encoding, this ends up cutting the overall metadata size
considerably (~1.5 megabytes on libcore). We can support both relative
and absolute encodings essentially for free since we already take a full
byte to differentiate between direct and indirect encodings (so an extra
variant is quite cheap).

Author: bors