[Glacier] Add the [] operator to string.

lib/glacier/string/string.cpp

@@ -38,4 +38,9 @@ bool String::operator==(const String& other) {
   return true;
 }
 
+char String::operator[](uint64_t offset) const {
+  // FIXME: bounds check.
+  return cstr_[offset];
+}
+
 }  // namespace glcr

lib/glacier/string/string.h

@@ -15,6 +15,8 @@ class String {
 
   bool operator==(const String& str);
 
+  char operator[](uint64_t offset) const;
+
  private:
   char* cstr_;
   uint64_t length_;