Add support for async for. Issue #124

Ignore 'boilerplate' opcodes that set up the async iteration logic internally (https://www.python.org/dev/peps/pep-0492/#asynchronous-iterators-and-async-for) - We don’t need any of this as GET_AITER is enough to tell us that we are in an `async for`.

ASTNode.cpp

@@ -1,4 +1,5 @@
 #include "ASTNode.h"
+#include <bytecode.h>
 
 /* ASTNodeList */
 void ASTNodeList::removeLast()
@@ -75,7 +76,45 @@ const char* ASTBlock::type_str() const
 {
     static const char* s_type_strings[] = {
         "", "if", "else", "elif", "try", "CONTAINER", "except",
-        "finally", "while", "for", "with",
+        "finally", "while", "for", "with", "async for"
     };
     return s_type_strings[blktype()];
 }
+
+
+/* ASTIterBlock */
+const int ASTIterBlock::ASYNCFOR_BOILER_READLOOPINDEX = 4;
+const int ASTIterBlock::ASYNCFOR_BOILER_READLOOPCONTENTS = 11;
+const int ASTIterBlock::ASYNCFOR_BOILER_OFFSETFROMEND = 12;
+const int ASTIterBlock::ASYNCFOR_BOILER_FIRSTJUMP = 5;
+const Pyc::Opcode ASTIterBlock::ASYNCFOR_BOILER_ALTJUMPOP = Pyc::JUMP_ABSOLUTE_A;
+
+const std::vector<Pyc::Opcode> ASTIterBlock::ASYNCFOR_BOILERPLATE = {
+    // Ignore all these after GET_AITER.
+    Pyc::SETUP_EXCEPT_A,
+    Pyc::GET_ANEXT,
+    Pyc::LOAD_CONST_A,          // None
+    Pyc::YIELD_FROM,
+
+    // Process ops from here to load index variable.
+
+    // Continue ignoring all these
+    Pyc::POP_BLOCK,
+    Pyc::JUMP_FORWARD_A,        // or JUMP_ABSOLUTE if pass
+    Pyc::DUP_TOP,
+    Pyc::LOAD_GLOBAL_A,         // StopAsyncIteration
+    Pyc::COMPARE_OP_A,          // (<EXCEPTION MATCH>)
+    Pyc::POP_JUMP_IF_TRUE_A,
+    Pyc::END_FINALLY,
+
+    // Actual code inside the loop begins here. 'Preamble' before this, 'Trailer' after.
+
+    // After reading a JUMP at the address we are expecting, ignore the following:
+    Pyc::POP_TOP,
+    Pyc::POP_TOP,
+    Pyc::POP_TOP,
+    Pyc::POP_EXCEPT,
+    Pyc::POP_TOP
+
+    // Then a POP_BLOCK as per normal loop
+};