documentation and small changes

- added docs for `Promise` and  `PromiseFactory`
- removed outdated README docs
- moved some common utilities to `PromiseUtil`
- improved efficiency of result array resizing
- added cancellation result to promise executors
- changed visibility of `PromiseJoiner` to public, and made some method names more verbose
- inlined `DeferredExecutionException` to inside `AbstractPromise`
- inlined default promise implementation to inner class in the factory
- removed necessity for base factories to provide a logger

futur-api/src/main/java/dev/tommyjs/futur/function/ExceptionalSupplier.java

@@ -1,8 +1,20 @@
 package dev.tommyjs.futur.function;
 
+/**
+ * Represents a supplier of results that may throw an exception.
+ * This is a functional interface whose functional method is {@link #get()}.
+ *
+ * @param <T> the type of results supplied by this supplier
+ */
 @FunctionalInterface
 public interface ExceptionalSupplier<T> {
 
+    /**
+     * Gets a result, potentially throwing an exception.
+     *
+     * @return a result
+     * @throws Exception if unable to supply a result
+     */
     T get() throws Exception;
 
-}
+}