Allow returning lua functions

Not sure how this will play with persistence when it happens (badly,
most likely), but it's not a bad idea to support it.

Closes #466

src/main/java/dan200/computercraft/api/lua/ILuaFunction.java

@@ -0,0 +1,29 @@
+/*
+ * This file is part of the public ComputerCraft API - http://www.computercraft.info
+ * Copyright Daniel Ratcliffe, 2011-2020. This API may be redistributed unmodified and in full only.
+ * For help using the API, and posting your mods, visit the forums at computercraft.info.
+ */
+package dan200.computercraft.api.lua;
+
+import javax.annotation.Nonnull;
+
+/**
+ * A function, which can be called from Lua. If you need to return a table of functions, it is recommended to use
+ * an object with {@link LuaFunction} methods, or implement {@link IDynamicLuaObject}.
+ *
+ * @see MethodResult#of(Object)
+ */
+@FunctionalInterface
+public interface ILuaFunction
+{
+    /**
+     * Call this function with a series of arguments. Note, this will <em>always</em> be called on the computer thread,
+     * and so its implementation must be thread-safe.
+     *
+     * @param arguments The arguments for this function
+     * @return The result of calling this function.
+     * @throws LuaException Upon Lua errors.
+     */
+    @Nonnull
+    MethodResult call( @Nonnull IArguments arguments ) throws LuaException;
+}

src/main/java/dan200/computercraft/api/lua/MethodResult.java

@@ -58,7 +58,7 @@ public final class MethodResult
      *
      * Integers, doubles, floats, strings, booleans, {@link Map}, {@link Collection}s, arrays and {@code null} will be
      * converted to their corresponding Lua type. {@code byte[]} and {@link ByteBuffer} will be treated as binary
-     * strings.
+     * strings. {@link ILuaFunction} will be treated as a function.
      *
      * In order to provide a custom object with methods, one may return a {@link IDynamicLuaObject}, or an arbitrary
      * class with {@link LuaFunction} annotations. Anything else will be converted to {@code nil}.