[SMTChecker] Implement uninterpreted functions and use it for blockhash()

test/libsolidity/smtCheckerTests/special/blockhash.sol

@@ -2,9 +2,13 @@ pragma experimental SMTChecker;
 
 contract C
 {
-	function f() public payable {
+	function f(uint x) public payable {
+		assert(blockhash(x) > 0);
 		assert(blockhash(2) > 0);
+		uint y = x;
+		assert(blockhash(x) == blockhash(y));
 	}
 }
 // ----
-// Warning: (79-103): Assertion violation happens here
+// Warning: (85-109): Assertion violation happens here
+// Warning: (113-137): Assertion violation happens here