fix: 'format_to_n' compiles 'std::back_inserter' arguments

std::back_insert_iterators model the OutputIterator concept but differ considerably in their traits and behavior. In particular the former made compilation to fail when format_to_n is given a back_inserter as first argument. The emulation of an OutputIterator is not perfect due to the behavioural differences of back_insert_iterators (e.g. assignment always implies increment) but good enough to be used within fmt's machinery.

test/format-test.cc

@@ -12,6 +12,7 @@
 #include <cstring>
 #include <list>
 #include <memory>
+#include <string>
 #include <stdint.h>
 
 // Check if fmt/format.h compiles with windows.h included before it.
@@ -185,6 +186,17 @@ TEST(IteratorTest, TruncatingIterator) {
   EXPECT_EQ(it.base(), p + 1);
 }
 
+TEST(IteratorTest, TruncatingBackInserter) {
+  std::string buffer;
+  auto bi = std::back_inserter(buffer);
+  fmt::internal::truncating_iterator<decltype(bi)> it(bi, 2);
+  *it++ = '4';
+  *it++ = '2';
+  *it++ = '1';
+  EXPECT_EQ(buffer.size(), 2);
+  EXPECT_EQ(buffer, "42");
+}
+
 TEST(MemoryBufferTest, Ctor) {
   basic_memory_buffer<char, 123> buffer;
   EXPECT_EQ(static_cast<size_t>(0), buffer.size());