General Chemistry examples: 'Lewis structures and resonance' becomes 'Resonance structures'; HCN replaces CN-

9.4.0 test (Andy): the topic has no Lewis structures. Keep N2O, NO3- and O3; rename the topic Resonance structures; BH3 and triplet CH2 leave it; CN- is replaced by HCN. See also #573 for the ionisation energy / electron affinity and Koopmans topics.

Decided (Andy 10-26): HCN goes to Polar bonds and dipole moments.