Implement memory.grow and memory.current (#9)

* Implement.

* Clean and doc

* Collect base addresses instead of leaking them

* Fix code for 1.25.

* Simplify $assert

* Use AbiParam::special.

* Use &mut self in base_addr

filetests/grow.wat

@@ -0,0 +1,28 @@
+(module
+  (memory 1)
+  (func $assert (param i32)
+    (block $ok
+      (br_if $ok
+        (get_local 0)
+      )
+      (unreachable)
+    )
+  )
+  (func $main (local i32)
+    (call $assert
+      (i32.eq
+        (grow_memory (i32.const 1))
+        (i32.const 1)
+      )
+    )
+    (call $assert
+      (i32.eq
+        (current_memory)
+        (i32.const 2)
+      )
+    )
+  )
+  (start $main)
+  (data (i32.const 0) "\04\03\02\01")
+)
+