Parse updated event text and binary format (#1014)
An event has a type-section index, like a function definition. The
current proposal doesn't specify the text format, so I assumed that it
would match the format of the other sections that reference function
types. This means that the following declaration styles are allowed:
```
(type $t (func (param i32)))
(event $e1 (type $t))
(event $e2 (param f32))
```
