Don't generate duplicate thunks

Previously, functions that appear in multiple places
(exported, start function, table) would generate a thunk
for each place they appear in. Those additional thunks are
identical and only only one of them would be referenced.
Main offender are tables with redundant entries.

This commit eliminates those duplicate thunks without adding
any additional overhead.

tests/expectations/stack-height/start.wat

@@ -22,23 +22,7 @@
     i32.const 1
     i32.sub
     global.set 0)
-  (func (;4;) (type 1)
-    global.get 0
-    i32.const 1
-    i32.add
-    global.set 0
-    global.get 0
-    i32.const 1024
-    i32.gt_u
-    if  ;; label = @1
-      unreachable
-    end
-    call 1
-    global.get 0
-    i32.const 1
-    i32.sub
-    global.set 0)
   (global (;0;) (mut i32) (i32.const 0))
-  (export "exported_start" (func 4))
+  (export "exported_start" (func 3))
   (export "call" (func 2))
-  (start 4))
+  (start 3))