Clojure interop from Java.

The clojure.java.api package provides a minimal interface to bootstrap Clojure access from other JVM languages. It does this by providing:

1. The ability to use Clojure's namespaces to locate an arbitrary var, returning the var's `clojure.lang.IFn` interface.
2. A convenience method `read` for reading data using Clojure's edn reader

`IFn`s provide complete access to Clojure's APIs. You can also access any other library written in Clojure, after adding either its source or compiled form to the classpath.

The public Java API for Clojure consists of the following classes and interfaces:

1. clojure.java.api.Clojure
2. clojure.lang.IFn

All other Java classes should be treated as implementation details, and applications should avoid relying on them.

To lookup and call a Clojure function:

    IFn plus = Clojure.var("clojure.core", "+");
    plus.invoke(1, 2);

Functions in `clojure.core` are automatically loaded. Other namespaces can be loaded via `require`:

    IFn require = Clojure.var("clojure.core", "require");
    require.invoke(Clojure.read("clojure.set"));

`IFn`s can be passed to higher order functions, e.g. the example below passes `plus` to `read`:

    IFn map = Clojure.var("clojure.core", "map");
    IFn inc = Clojure.var("clojure.core", "inc");
    map.invoke(inc, Clojure.read("[1 2 3]"));

Most IFns in Clojure refer to functions. A few, however, refer to non-function data values. To access these, use `deref` instead of `fn`:

    IFn printLength = Clojure.var("clojure.core", "*print-length*");
    IFn deref = Clojure.var("clojure.core", "deref");
    deref.invoke(printLength);