feat(furvm): introduce length of operator

Closes: #54

furvm/include/furvm/instruction.hpp

@@ -125,6 +125,11 @@ enum class instruction_t : byte {
      */
     Sizeof,
 
+    /**
+     * @brief Pushes a length of popped-off thing onto the stack.
+     */
+    Lengthof,
+
     /**
      * @brief Pushes a variable onto the stack.
      *

furvm/include/furvm/thing.hpp

@@ -453,7 +453,7 @@ public:
         return { *m_type.value.array.type, data + (index * elementSize), m_allocator };
     }
 
-    std::size_t size() const {
+    thing_type::u64 length() const {
         if (m_type.type != thing_type::Array) throw bad_thing_access();
         return m_type.value.array.size == 0 ? get<array>().dynamic.size : m_type.value.array.size;
     }