Define some tests for the type-checker

src/test/type-inference.md

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+
+## Record types can be unified without causing an error
+
+```
+struct Person.
+  email: String
+  age: Int
+
+let bert = Person { email = "bar@boo.com", age = 32 }
+let bob = Person { email = "boo", age = 43 }
+
+bert == bob
+```
+
+## Return types are polymorphic
+
+```
+let id x = x
+
+id 1
+id "foo"
+id True
+```
+
+## Nested definitions work
+
+```
+let foo x.
+  let bar y z = y + z - x
+  bar
+
+foo True
+```
+
+## Everything that can be type-checked will be type-checked
+
+```
+let foo n.
+  let f : String = 1
+  return n
+```
+
+## Recursive definitions do not cause infinite loops in the type-checker
+
+```
+let fac n = fac_2 n
+
+let fac_2 n = fac_3 n + fac n
+
+let fac_3 n = fac_2 (n-1)
+
+not (fac 1)
+```
+
+## Example with mutual recursion works
+
+```
+let is_even x.
+  if x == 0.
+    return True
+  else.
+    return is_odd (x-1)
+
+let is_odd x.
+  if x == 1.
+    return False
+  else.
+    return is_even (x-1)
+
+not (is_even True)
+```