use C style comments instead of C++ style comments

According to CONTRIBUTING.md,

Don't use C++ style comments

 /* This is the prefered comment style */

Use C++ style comments only for temporary comment e.g. commenting out some code lines.

include/mruby/dump.h

@@ -55,17 +55,17 @@ mrb_irep *mrb_read_irep(mrb_state*, const uint8_t*);
 
 #define MRB_DUMP_DEFAULT_STR_LEN      128
 
-// binary header
+/* binary header */
 struct rite_binary_header {
-  uint8_t binary_identify[4]; // Binary Identifier
-  uint8_t binary_version[4];  // Binary Format Version
-  uint8_t binary_crc[2];      // Binary CRC
-  uint8_t binary_size[4];     // Binary Size
-  uint8_t compiler_name[4];   // Compiler name
+  uint8_t binary_identify[4]; /* Binary Identifier */
+  uint8_t binary_version[4];  /* Binary Format Version */
+  uint8_t binary_crc[2];      /* Binary CRC */
+  uint8_t binary_size[4];     /* Binary Size */
+  uint8_t compiler_name[4];   /* Compiler name */
   uint8_t compiler_version[4];
 };
 
-// section header
+/* section header */
 #define RITE_SECTION_HEADER \
   uint8_t section_identify[4]; \
   uint8_t section_size[4]
@@ -77,7 +77,7 @@ struct rite_section_header {
 struct rite_section_irep_header {
   RITE_SECTION_HEADER;
 
-  uint8_t rite_version[4];    // Rite Instruction Specification Version
+  uint8_t rite_version[4];    /* Rite Instruction Specification Version */
 };
 
 struct rite_section_lineno_header {

include/mruby/khash.h

@@ -24,7 +24,7 @@ typedef khint_t khiter_t;
 
 #define UPPER_BOUND(x) ((x)>>2|(x)>>1)
 
-//extern uint8_t __m[];
+/* extern uint8_t __m[]; */
 
 /* mask for flags */
 static const uint8_t __m_empty[8]  = {0x02, 0x08, 0x20, 0x80};

include/mruby/variable.h

@@ -16,9 +16,9 @@ typedef struct global_variable {
     mrb_value *data;
     mrb_value (*getter)(void);
     void  (*setter)(void);
-    //void  (*marker)();
-    //int block_trace;
-    //struct trace_var *trace;
+    /* void  (*marker)(); */
+    /* int block_trace; */
+    /* struct trace_var *trace; */
 } global_variable;
 
 struct global_entry {