Function: easy-menu-convert-item

Memoize the value returned by `easy-menu-convert-item-1' called on ITEM.
This makes key-shortcut-caching work a *lot* better when this
conversion is done from within a filter.
This also helps when the NAME of the entry is recreated each time:
since the menu is built and traversed separately, the lookup
would always fail because the key is `equal' but not `eq'.