Generate `is_XYZ` methods for events.

It's often the case that we want to extract _a_ variant (subset) out of list of events. Right now we have to pattern match, add one clause for `true` and a wildcard for `false`:
```rust
get_events(contract_events)
.filter(|event| {
match event {
pair_contract::event::Event::Mint { .. } => true,
_ => false,
}
})
.collect()
```

with methods like `Event::is_mint` that code will be much terser.