A single-entry cache for a SpanData, to optimize a particular scenario: when iterating
through child spans (e.g. during encoding/decoding/hashing), many of those spans will have the
same parent span. In the case where that parent span is interned (e.g. the parent is a const
literal item exceeding MAX_LEN bytes of source code), this cache avoids having to fetch the
parent’s SpanData from the interner (which involves TLS, locking, etc.) over and over. A
single entry is enough for an extremely high hit rate.
This is the context state available during incr. comp. hashing. It contains
enough information to transform DefIds and HirIds into stable DefPaths (i.e.,
a reference to the TyCtxt) and it holds a few caches for speeding up various
things (e.g., each DefId/DefPath is only hashed once).