Add log-scope, log-entry and log-exit

Closes #3
Add stage to add signature to :doc meta

The add-sig-doc stage adds any :sig metadata to the function's doc string.

Closes #1
Refactor namespaces

Introduce com.palletops.api-builder.core which can be used from
com.palletops.api-builder and com.palletops.api-builder.stage*
without causing circular dependencies
Add a defn-api that uses all the stages

Closes #2
Update readme
Add api-defn to readme