Invoke `_initialize` method on every call if present.
In NEO3 there is no entrypoint and methods are invoked by offset,
thus `Main` function is no longer required.
We still have special `Main` method in tests to simplify them.
When `return` or `break` statement is encountered inside
a for/range/switch statement, top stack items can be auxilliary.
They need to be cleaned up before returning from the function.
These don't belong to VM as they compile some Go code and run it in a VM. One
may call them integration tests, but I prefer to attribute them to
compiler. Moving these tests into pkg/compiler also allows to properly count
the compiler coverage they add:
-ok github.com/CityOfZion/neo-go/pkg/compiler (cached) coverage: 69.7% of statements
+ok github.com/CityOfZion/neo-go/pkg/compiler (cached) coverage: 84.2% of statements
This change also fixes `contant` typo and removes fake packages exposed to the
public by moving foo/bar/foobar into the testdata directory.
