Improvements to docs

[nicoburns]

• Simplify public module hierarchy (make some modules private and re-export types instead)
• Re-export all types at the top level (making use of wildcard re-exports to avoid cluttering the docs)
• Add a usage example and basic documentation to lib.rs

[nicoburns]

please make sure that this doesn't have warnings or (new) clippy lints with a current nightly from your lifetime changes.

The lifetime changes were to make nightly clippy happy in the first place!