Export types

[majelbstoat]

/src/core.d.ts helpfully has Typescript types for things like BunyanLog and HTTPRequest. Unfortunately, even though they're imported into index.d.ts, they're not exported, which means we can't use them for strong typing in userland.

Would you consider exporting types so we don't have to redefine them?

[majelbstoat]

... and not 10 minutes later I realised I could do this:

import { BunyanLogRecord, HttpRequest } from '@google-cloud/logging-bunyan/build/src/types/core'