more optimizations that v1 does and some cleanups

- a = a + x ==> a+=x
- joining consecutive var statements (hoisting is not always desirable)
- x == false ==> x == 0, x != true ==> x != 1
- x, x ==> x; x = exp(), x ==> x = exp()
- discarding useless break-s

tmp/todo

@@ -1,11 +1,11 @@
-a = a + x ==> a+=x
+✓ a = a + x ==> a+=x
 
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-join consecutive var statements
+✓ join consecutive var statements
 
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-
+✓
 x == false ==> x == 0
 x == true  ==> x == 1
 
@@ -14,6 +14,8 @@ JS is so sloppy that this could be an indication of a bug.
 
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+✓
+
 x, x ==> x
 x = foo, x ==> x
 
@@ -30,6 +32,8 @@ XXX?  Not sure if this is worth the trouble.
 
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+✓
+
 discard spurious break statements
 
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