Event API: add Map.compute-like methods

Add Map-like `compute`, `computeIfAbsent`, and `computeIfPresent`:

Two flavors are possible:

* source + target: `Event.compute(FieldReference source, FieldReference target, Function compute)`
* modify the field: `Event.compute(FieldReference field, Function compute)`

Mostly the positive impact is believed (per discussion with a few folks) to be around the mutate filter and script/ruby filters. For this reason, the `modify the field` option is probably a good target since we want to enable functionality with less typing for script/ruby filter users.

Proposal:

```
void compute(FieldReference, field, Function compute) throws FieldMissingException;

// most valuable, probably, for script/ruby filter users
void computeIfPresent(FieldReference, field, Function compute) throws FieldMissingException;

void computeIfAbsent(FieldReference, field, Function compute) throws FieldMissingException;
```