[lang] REmove @Pure on map extension functions.

see #257

Signed-off-by: Stéphane Galland <galland@arakhne.org>
[lang] Remove conflicting operations in MapExtensions.

The (map->key) and (pair + map) operators are under conflict with other
implicit extension functions.

Signed-off-by: Stéphane Galland <galland@arakhne.org>