Export AllEvents type for Truffle target

[wbt]

With the Truffle target, a type 'AllEvents' is defined but there doesn't seem to be a way to access it externally, though there does seem to be a way to access the set of events for a web3 target (see #249). Adding the 'export' keyword before 'type AllEvents' seems like the simplest way to solve this.

[wbt]

An exported map of the event names to the types would likely be better.