Enhance log experience

In #30, we landed initial support for being a log target. But we can go further.

Ideas:

- Add custom log macros (`trace`, `info`, `debug`, `warn`, `error`) that take a (mutable ref to an) `Output` as first param
- Behind a cargo feature called "log-macros" (or similar)
- Be compatible with [slog](https://docs.rs/slog) and support its many adapters
- Behind a cargo feature called "slog"
- Look at https://github.com/rust-lang-nursery/log/pull/296 -- structured data in log macros

Start by reviewing the implementation landed in #30 and the current Output API. Then compare the proposed log macros, Cargo feature boundaries, slog compatibility, and structured-data direction in rust-lang-nursery/log#296; done requires settling which of these ideas belong in the scope and defining acceptance criteria.

Written by the indexing model from the issue text.