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- removed outdated README docs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/promise/PromiseCompletion.java

@@ -5,39 +5,78 @@ import org.jetbrains.annotations.Nullable;
 
 import java.util.concurrent.CancellationException;
 
+/**
+ * Represents the result of a {@link Promise}, containing either an optional result or an exception.
+ */
 public class PromiseCompletion<T> {
 
     private @Nullable T result;
     private @Nullable Throwable exception;
 
+    /**
+     * Creates a new successful completion.
+     * @param result the result
+     */
     public PromiseCompletion(@Nullable T result) {
         this.result = result;
     }
 
+    /**
+     * Creates a new exceptional completion.
+     * @param exception the exception
+     */
     public PromiseCompletion(@NotNull Throwable exception) {
         this.exception = exception;
     }
 
+    /**
+     * Creates a new successful completion with a result of {@code null}.
+     */
     public PromiseCompletion() {
-        this.result = null;
+        this((T) null);
     }
 
+    /**
+     * Checks if the completion was successful.
+     * @return {@code true} if the completion was successful, {@code false} otherwise
+     */
     public boolean isSuccess() {
         return exception == null;
     }
 
+    /**
+     * Checks if the completion was exceptional.
+     * @return {@code true} if the completion was exceptional, {@code false} otherwise
+     */
     public boolean isError() {
         return exception != null;
     }
 
-    public boolean wasCanceled() {
+    /**
+     * Checks if the completion was cancelled.
+     * @return {@code true} if the completion was cancelled, {@code false} otherwise
+     */
+    public boolean wasCancelled() {
         return exception instanceof CancellationException;
     }
 
+    @Deprecated
+    public boolean wasCanceled() {
+        return wasCancelled();
+    }
+
+    /**
+     * Gets the result of the completion.
+     * @return the result, or {@code null} if the completion was exceptional
+     */
     public @Nullable T getResult() {
         return result;
     }
 
+    /**
+     * Gets the exception of the completion.
+     * @return the exception, or {@code null} if the completion was successful
+     */
     public @Nullable Throwable getException() {
         return exception;
     }