Improve performance of H5Ovisit with deeply nested groups (#6272)

Improve performance of H5Ovisit (and H5Ocopy, and functions that
retrieve and object name) by passing more information about the visited
object from the underlying H5G_visit routine to these callbacks.
Introduced an internal object callback for H5G_visit to facilitate this.
H5Ovisit1 and potentially H5Ovisit2 are still slow with deeply nested
groups due to the way these deprecated functions interact with the VOL
layer.

release_docs/CHANGELOG.md

@@ -65,6 +65,10 @@ We would like to thank the many HDF5 community members who contributed to this r
 
 ## Library
 
+### Improve performance of H5Ovisit() with deeply nested group structures
+
+   `H5Ovisit()` would previously internally traverse each object's path name from the iteration root group in order to retrieve information about that object, causing severe performance degradation with a deeply nested group structure. Modified the algorithm to instead retrieve information directly from the object. To get this benefit, users should use `H5Ovisit3()`, or use `H5Ovisit2()` with neither `H5O_INFO_HDR` nor `H5O_INFO_META_SIZE` selected in the `fields` parameter. Performance of `H5Ocopy()`, `H5Iget_name()`, and external links with a callback set should also improve in similar situations.
+
 ## Parallel Library
 
 ## Fortran Library