Speed-up dictionary constructor by about 10%.

New opcode, STORE_MAP saves the compiler from awkward stack manipulations
and specializes for dicts using PyDict_SetItem instead of PyObject_SetItem.

Old disassembly:
              0 BUILD_MAP                0
              3 DUP_TOP
              4 LOAD_CONST               1 (1)
              7 ROT_TWO
              8 LOAD_CONST               2 ('x')
             11 STORE_SUBSCR
             12 DUP_TOP
             13 LOAD_CONST               3 (2)
             16 ROT_TWO
             17 LOAD_CONST               4 ('y')
             20 STORE_SUBSCR

New disassembly:
              0 BUILD_MAP                0
              3 LOAD_CONST               1 (1)
              6 LOAD_CONST               2 ('x')
              9 STORE_MAP
             10 LOAD_CONST               3 (2)
             13 LOAD_CONST               4 ('y')
             16 STORE_MAP
diff --git a/Include/opcode.h b/Include/opcode.h
index 2816369..9f20b56 100644
--- a/Include/opcode.h
+++ b/Include/opcode.h
@@ -45,6 +45,7 @@
 #define DELETE_SLICE	50
 /* Also uses 51-53 */
 
+#define STORE_MAP	54
 #define INPLACE_ADD	55
 #define INPLACE_SUBTRACT	56
 #define INPLACE_MULTIPLY	57