feat: add <<< and >>> abbreviating operations to default stream

Resolves #16.

tests/basic.spt

@@ -231,3 +231,15 @@ run desugar-fostr to TopLevel(Sequence([
   Terminate(Sum(Int("11"), To(Int("12"), Stream()))),
   To(To(Int("13"), Stream()), Stream())
 ]))
+
+test emit_several_default [[
+<<< 1 + 2; 3 >>>
+(4 + 5) >>> >> stream; stream << 6;
+<<< 7 << 75
+<<< 8
+  + (9+10);
+11 + 12 >>>; 13 >>>
+                >>>
+]] parse succeeds
+/** writes
+3399677527121313*/

tests/emit_several.fos

@@ -1,7 +1,7 @@
-stream << 1 + 2; 3 >> stream
-(4 + 5) >> stream; stream << 6;
-stream << 7 
-stream << 8
+<<< 1 + 2; 3 >>>
+(4 + 5) >>> >> stream; stream << 6;
+<<< 7 << 75
+<<< 8
   + (9+10);
-11 + 12 >> stream; 13 >> stream
-                      >> stream
+11 + 12 >>>; 13 >>>
+                >>>

tests/hw.fos

@@ -1 +1 @@
-stream << 'Hello,	world!'
+<<< 'Hello, world!'