Separate memory management and formatting

Array is split into an abstract Buffer class and a concrete MemoryBuffer class. BasicWriter now does all memory allocation through a Buffer object. Subclasses of BasicWriter may use different buffer types. The new BasicMemoryBuffer class uses the default MemoryBuffer.

test/gtest-extra-test.cc

@@ -348,7 +348,7 @@ TEST(StreamingAssertionsTest, EXPECT_WRITE) {
 }
 
 TEST(UtilTest, FormatSystemError) {
-  fmt::Writer out;
+  fmt::MemoryWriter out;
   fmt::internal::format_system_error(out, EDOM, "test message");
   EXPECT_EQ(out.str(), format_system_error(EDOM, "test message"));
 }