* Urban decay grips Zimbabwe's capital, Harare
* Less than 20 pct in formal employment
* Informal economy becoming the new economic model
By MacDonald Dzirutwe
HARARE, April 3 (Reuters) - On Harare's hardscrabble streets, college graduates compete with peasants scratching out a living selling anything from mobile phone cards to herbal sex tonics, a measure of the decline of Zimbabwe's ...