Copy the leaf or subtree at segments from source into the same nested
path in out, creating intermediate objects as needed; an absent or
non-object path is skipped.
Split one event_fields entry into its segments, inverting the MSC3873
escape grammar used by rumaโs escape_key: \. is a literal dot inside the
current segment, \\ a literal backslash, any other \X is kept verbatim,
and an unescaped . ends the segment.