Announce 1.0.3
Move SyntaxProcessor to another package
Candidate: SchemaKey

The idea is to have a unique key for JSON Schemas, whether anonymous or
"located" (ie, with an absolute JSON Reference), with a fast .equals() and
.hashCode() so as to be useable as, for instance, keys in a HashSet.

For this, we separate two cases:

* anonymous schemas; they have no location, but we can generate a unique id for
  them;
* "URI-located" schemas; they do have a location.

Each of these two cases are implemented in two distinct classes. The way
`.equals()` is implemented for each of them, we are also guaranteed that an
anonymous schema key will never be equal to a URI-located schema key.
Rename KeywordDescriptor to Keyword
Keyword: do allow a null syntax checker and non null collector
Message cleanup
Keyword: add a Digester
Remove unused classes
Optimize imports
SchemaTree: remove .getId()

Superseded by SchemaKey.
Remove one test file...

... Which triggered a compile failure. No idea why... IDEA... didn't notice
that.
BaseSchemaTree: add a SchemaKey member
Delete obsolete class
AsJson: make it extend JsonSerializable

The end goal is to get rid of AsJson completely. Start and make its `.asJson()`
method obsolete.
New class JsonUtils

Somewhat misnamed. Its intent is to serialize arbitrary objects to various JSON
representations.

Start with a .toJson() method which will serialize an object either to its
JsonSerializable form if it implements it, or to a condensed digest (outputting
the full Java class name).
ValueHolder: make final

Ultimate goal is to remove it. This is the first step.
JsonUtils: add more utility methods
JsonUtils: make arguments @Nullable

If arguments are null, a NullNode is returned.
ProcessingMessage: remove deprecated methods
AbstractSyntaxChecker: resolve one javadoc issue