Add readers for ClassDefinition and TypeParam

The reason why we add the new `Modifiers` constructor is to have an
efficient way to instantiate `Modifiers` without passing bit per bit
which flags are set.

The new constructor is `protected` and hence the user of it in
`converters` (`InternalApiProxy`) is placed in the `xsbti.api` package.
Comments and docs have been added to ensure that no downstream user
depends on it.

Author: jvican

src/main/java/xsbti/api/Modifiers.java

@@ -37,6 +37,25 @@ public Modifiers(boolean isAbstract, boolean isOverride, boolean isFinal, boolea
 		);
 	}
 
+	/**
+     * Allow to set the modifiers from a flags byte where:
+	 *
+	 *   1. The first bit tells if has an abstract modifier.
+	 *   2. The first bit tells if has an override modifier.
+	 *   3. The first bit tells if has an final modifier.
+	 *   4. The first bit tells if has an sealed modifier.
+	 *   5. The first bit tells if has an implicit modifier.
+	 *   6. The first bit tells if has an lazy modifier.
+	 *   7. The first bit tells if has an macro modifier.
+	 *   8. The first bit tells if has an super accessor modifier.
+	 *
+	 * This method is not part of the public API and it may be removed at any point.
+	 * @param flags An instance of byte encoding the modifiers.
+	 */
+	protected Modifiers(byte flags) {
+		this.flags = flags;
+	}
+
 	private final byte flags;
 
 	private boolean flag(int bit)