Add ceiling to NumberInterface and the two numbers that implement it.

src/main/java/org/nwapw/abacus/number/NaiveNumber.java

@@ -93,6 +93,11 @@ public class NaiveNumber implements NumberInterface {
         return this.compareTo(ZERO);
     }
 
+    @Override
+    public int ceiling() {
+        return (int) Math.ceil(value);
+    }
+
     @Override
     public NumberInterface promoteTo(Class<? extends NumberInterface> toClass) {
         if (toClass == this.getClass()) return this;

src/main/java/org/nwapw/abacus/number/NumberInterface.java

@@ -79,6 +79,12 @@ public interface NumberInterface {
      */
     int signum();
 
+    /**
+     * Returns the least integer greater than or equal to the number.
+     * @return the least integer >= the number, if int can hold the value.
+     */
+    int ceiling();
+
     /**
      * Promotes this class to another number class.
      *

src/main/java/org/nwapw/abacus/number/PreciseNumber.java

@@ -1,6 +1,7 @@
 package org.nwapw.abacus.number;
 
 import java.math.BigDecimal;
+import java.math.MathContext;
 import java.math.RoundingMode;
 
 public class PreciseNumber implements NumberInterface {
@@ -49,7 +50,7 @@ public class PreciseNumber implements NumberInterface {
 
     @Override
     public NumberInterface multiply(NumberInterface multiplier) {
-        return new PreciseNumber(value.multiply(((PreciseNumber) multiplier).value));
+        return new PreciseNumber(this.value.multiply(((PreciseNumber) multiplier).value));
     }
 
     @Override
@@ -94,6 +95,11 @@ public class PreciseNumber implements NumberInterface {
         return value.signum();
     }
 
+    @Override
+    public int ceiling() {
+        return (int) Math.ceil(value.doubleValue());
+    }
+
     @Override
     public NumberInterface negate() {
         return new PreciseNumber(value.negate());