pprint properly prints hash-maps

[jig]

Current implementation of pprint is not compatible with implementations using languages that do not guarantee that keys on a hash-map keep ordering (e.g. Go language).

This PR corrects this.

[jig]

With previous code, the effect was that some keys appear duplicated and others do not appear at all when printing a hash-map with pprint.

This fix extracts the keys list only once, so hash-map keys order indetermination is not an issue.

Validated with Go language.