Add StringStream implementation to std (#721)

std/text/stringstream.spice

@@ -0,0 +1,74 @@
+/**
+ * Helper data structure to construct strings.
+ * It is useful to avoid copying large strings around.
+ */
+public type StringStream struct {
+    String buffer
+}
+
+public p StringStream.ctor() {
+    // Initialize buffer empty
+    this.buffer = String();
+}
+
+public p StringStream.ctor(string input) {
+    // Initialize buffer with an initial string
+    this.buffer = String(input);
+}
+
+public p StringStream.ctor(const String& input) {
+    // Initialize buffer with an initial string
+    this.buffer = input;
+}
+
+/**
+ * Clear the buffer
+ */
+public p StringStream.clear() {
+    this.buffer.clear();
+}
+
+/**
+ * Get the value of the buffer
+ *
+ * @return Resulting String object
+ */
+public f<const String&> StringStream.str() {
+    return this.buffer;
+}
+
+/**
+ * Append a raw string to the buffer
+ */
+#[ignoreUnusedReturnValue]
+public f<StringStream&> operator<<(StringStream& ss, string input) {
+    ss.buffer += input;
+    return ss;
+}
+
+/**
+ * Append a String to the buffer
+ */
+#[ignoreUnusedReturnValue]
+public f<StringStream&> operator<<(StringStream& ss, const String& input) {
+    ss.buffer += input;
+    return ss;
+}
+
+/**
+ * Append a single char to the buffer
+ */
+#[ignoreUnusedReturnValue]
+public f<StringStream&> operator<<(StringStream& ss, char input) {
+    ss.buffer += input;
+    return ss;
+}
+
+/**
+ * Return end line character
+ *
+ * @return End line character
+ */
+public f<char> endl() {
+    return '\n';
+}

test/test-files/std/text/stringstream/cout.out

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+All assertions passed!

test/test-files/std/text/stringstream/source.spice

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+import "std/text/stringstream";
+
+f<int> main() {
+    // Basic usage with raw string literals
+    StringStream ss1;
+    ss1 << "Hello" << " World!";
+    assert ss1.str() == "Hello World!";
+
+    // Basic usage with String values
+    StringStream ss2 = StringStream(String("This "));
+    ss2 << String("is") << String(" a ") << String("test!") << endl();
+    assert ss2.str() == "This is a test!\n";
+
+    // Mixed usage
+    StringStream ss3 = StringStream("Hello");
+    ss3 << String(" fellow") << " Programmers" << endl();
+    assert ss3.str() == "Hello fellow Programmers\n";
+
+    // Usage spread over multiple lines
+    StringStream ss4 = StringStream();
+    ss4 << "This ";
+    ss4 << "is ";
+    ss4 << "a ";
+    ss4 << "multiline ";
+    ss4 << "test";
+    ss4 << endl();
+    assert ss4.str() == "This is a multiline test\n";
+
+    printf("All assertions passed!");
+}