typecheck: infer typed parameter return expressions

tests/typecheck_function_return_expression_bad.claro

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+TEACH label amount RETURNS TEXT
+    CHECK TYPE amount IS NUMBER
+    RETURN amount + 1
+END

tests/typecheck_function_return_expression_good.claro

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+TEACH square amount RETURNS NUMBER
+    CHECK TYPE amount IS NUMBER
+    RETURN amount + 1
+END