There is no need for include_once() and require_once()

Aer is object-oriented language and the code it includes is executed in the global scope. Global scope can contain only classes and namespaces and they cannot be re-declared. Thus every file can be included just once. We can
leave this to programmer, or take care about it on interpreter side. With this commit, include() and require() builtin functions allows to load file just once. Thanks to that, there is no need for include_once() and
require_once(). Done in scope of #30.

include/ph7int.h

@@ -1455,7 +1455,6 @@ enum ph7_expr_id {
 #define PH7_KEYWORD_SWITCH       3 /* switch */
 #define PH7_KEYWORD_INTERFACE    5 /* interface */
 /* The number '8' is reserved for PH7_TK_ID */
-#define PH7_KEYWORD_REQONCE      9 /* require_once */
 #define PH7_KEYWORD_REQUIRE      10 /* require */
 #define PH7_KEYWORD_ELIF         0x4000000 /* elseif: MUST BE A POWER OF TWO */
 #define PH7_KEYWORD_ELSE         0x8000000 /* else:  MUST BE A POWER OF TWO */
@@ -1485,7 +1484,6 @@ enum ph7_expr_id {
 #define PH7_KEYWORD_EXIT         35 /* exit */
 #define PH7_KEYWORD_FINALLY      36 /* finally */
 #define PH7_KEYWORD_IMPLEMENTS   39 /* implements */
-#define PH7_KEYWORD_INCONCE      40 /* include_once */
 #define PH7_KEYWORD_INCLUDE      41 /* include */
 #define PH7_KEYWORD_EMPTY        42 /* empty */
 #define PH7_KEYWORD_INSTANCEOF   0x800 /* instanceof: MUST BE A POWER OF TWO  */