Add semantic tests using stringutils

test/libsolidity/semanticTests/externalContracts/strings.sol

@@ -0,0 +1,76 @@
+==== ExternalSource: _stringutils/stringutils.sol ====
+==== Source: strings.sol ====
+pragma abicoder v2;
+import "_stringutils/stringutils.sol";
+
+contract test {
+    using strings for bytes32;
+    using strings for string;
+    using strings for strings.slice;
+
+    function toSlice(string memory a) external pure returns (strings.slice memory) {
+        return a.toSlice();
+    }
+
+    function roundtrip(string memory a) external pure returns (string memory) {
+        return a.toSlice().toString();
+    }
+
+    function utf8len(string memory a) external pure returns (uint) {
+        return a.toSlice().len();
+    }
+
+    function multiconcat(string memory a, uint count) public pure returns (string memory) {
+        strings.slice memory s = a.toSlice();
+        for (uint i = 0; i < count; i++) {
+            s = s.concat(s).toSlice();
+        }
+        return s.toString();
+    }
+
+    function benchmark(string memory text, bytes32 seed) external pure returns (uint) {
+        // Grow text.
+        text = multiconcat(text, 10);
+
+        strings.slice memory a = text.toSlice();
+        strings.slice memory b = seed.toSliceB32();
+
+        // Some heavy computation.
+        bool c = b.equals(a) || b.startsWith(a);
+
+        // Join as a list.
+        strings.slice memory delim = c ? string(",").toSlice() : string(";").toSlice();
+        strings.slice[] memory parts = new strings.slice[](2);
+        parts[0] = a;
+        parts[1] = b;
+        string memory d = delim.join(parts);
+        return d.toSlice().len();
+    }
+}
+// ====
+// compileViaYul: also
+// ----
+// constructor()
+// gas irOptimized: 926170
+// gas legacy: 1192776
+// gas legacyOptimized: 777238
+// toSlice(string): 0x20, 11, "hello world" -> 11, 0xa0
+// gas irOptimized: 22877
+// gas legacy: 23190
+// gas legacyOptimized: 22508
+// roundtrip(string): 0x20, 11, "hello world" -> 0x20, 11, "hello world"
+// gas irOptimized: 23783
+// gas legacy: 24560
+// gas legacyOptimized: 23481
+// utf8len(string): 0x20, 16, "\xf0\x9f\x98\x83\xf0\x9f\x98\x83\xf0\x9f\x98\x83\xf0\x9f\x98\x83" -> 4 # Input: "😃😃😃😃" #
+// gas irOptimized: 24779
+// gas legacy: 25716
+// gas legacyOptimized: 24115
+// multiconcat(string,uint256): 0x40, 3, 11, "hello world" -> 0x20, 0x58, 0x68656c6c6f20776f726c6468656c6c6f20776f726c6468656c6c6f20776f726c, 0x6468656c6c6f20776f726c6468656c6c6f20776f726c6468656c6c6f20776f72, 49027192869463622675296414541903001712009715982962058146354235762728281047040 # concatenating 3 times #
+// gas irOptimized: 30478
+// gas legacy: 36801
+// gas legacyOptimized: 30420
+// benchmark(string,bytes32): 0x40, 0x0842021, 8, "solidity" -> FAILURE, hex"4e487b71", 0x11
+// gas irOptimized: 27298
+// gas legacy: 31621
+// gas legacyOptimized: 27668