add 'g' and 'G' commands to go to first and last line in buffer

src/core/Buffer.h

@@ -19,7 +19,7 @@ public:
             INVALID_CODE_POINT = 0xFFFFFFFFu,
         };
 
-        Iterator(Buffer * buffer)
+        Iterator(const Buffer * buffer)
         {
             m_buffer = buffer;
             m_offset = 0u;
@@ -87,7 +87,7 @@ public:
         size_t offset() const { return m_offset; }
 
     protected:
-        Buffer * m_buffer;
+        const Buffer * m_buffer;
         size_t m_offset;
         size_t m_line;
     };
@@ -114,6 +114,8 @@ public:
     void insert_code_point(uint32_t code_point, bool adjust_iterators = true);
     void erase_code_point(const Buffer::Iterator & position);
     std::string filename() const { return m_filename; }
+    Iterator begin() const { return Iterator(this); }
+    Iterator end() const { return *m_eof_iterator; }
 
 protected:
     bool m_eol_at_eof;