No tests added for the new types yet, I'll get on that tomorrow.

Motivation
---

The connection between the trigger event and the result types can be
confusing sometimes - for example `CloudFormationCustomResourceResponse`
is *not* returned from the lambda, but instead should be sent to the
*Request `ResponseUrl`. This PR tries to help by providing pre-baked
`Handler` types for each of the triggers (that have types already).

There's also a few niggles with naming and the handler signature that
have been bothering me for a while.

Changes
---

This PR:

- Adds defaulted generic parameters to the existing `Handler` and
  `Callback` types. This increases the minimum TS version to 2.3.

- Makes the `callback` parameter to `Handler`s "required" - this
  probably originally was meant to represent the Node 0.10 runtime,
  which did not pass a callback parameter, but that is no longer
  selectable when creating or editing a Lambda and makes the normal,
  recommended usage harder. In case anyone is confused, this doesn't
  break any code: it is required to be provided when being called, your
  handlers don't need to declare it. I'm not removing the legacy node 0.10
  runtime methods `context.done()` etc., since they still work.

- In the spirit of DefinitelyTyped#21874, make the default result type for
  `Handler`, `Callback` `any`, since the only requirement is
  that it be `JSON.stringify()`able, and there are things like
  `.toJSON()`, etc. so there is no meaningful type restriction
  here.

- Revert the definition changes of DefinitelyTyped#22588, which changed the `Handler`
  result types to `void | Promise<void>`, which is misleading: Lambda
  will not accept or wait for a promise result (it by default waits
  for the node event loop to empty). This is not a breaking change for
  code that does return promises, since a `void` result type is
  compatible with any result type, even with strict function types.
  (Which is why the tests still pass.)

- Adds `FooHandler` and `FooCallback` types for all `FooEvent`s and
  `FooResult`s, where possible - sometimes these don't match up.

- Renamed, with aliases to the old name, various types to align them: in
  particular `ProxyResult` -> `APIGatewayProxyResult`.

- `CloudFrontResult` (not the same type as `CloudFrontResponse`!)
  was missing for some reason.