Move clojure code to src/clojure
Java classes for marking Yaml data with source positions.

MarkedConstructor is a total hack but I think the best way to do it.
`make-yaml` knows how to use MarkedConstructor..
Some changes to parse-string.

`keywordize` is now a keyword argument, added `unsafe` and `mark` keyword
arguments.
:keywordize -> :keywords
A nicer Marked type.

This is decoded from our Java Marked type ; I'm actively looking for
a better way to do this, but this works for now.

Also added some tests for source positions, now that we have a nicer
way to deal with Marked.
Make Marked a static member of MarkedConstructor

This is marginally nicer.
Refactorings + docstrings.
Refactored MarkedConstructor.
Bump version minor number.