Move userspace to a templated StrFormat.

lib/glacier/string/string_builder.cpp

@@ -0,0 +1,24 @@
+#include "glacier/string/string_builder.h"
+
+namespace glcr {
+
+void StringBuilder::PushBack(const StringView& str) {
+  if (capacity() < size() + str.size()) {
+    uint64_t new_capacity = capacity() == 0 ? 1 : capacity() * 2;
+    while (new_capacity < size() + str.size()) {
+      new_capacity *= 2;
+    }
+    Resize(new_capacity);
+  }
+  for (uint64_t i = 0; i < str.size(); i++) {
+    Vector<char>::PushBack(str[i]);
+  }
+}
+
+String StringBuilder::ToString() const { return String(RawPtr(), size()); }
+
+StringBuilder::operator StringView() const {
+  return StringView(RawPtr(), size());
+}
+
+}  // namespace glcr