Function step_back

src/toml.hpp:12684–12697

		}

		TOML_NODISCARD
		const utf8_codepoint* step_back(size_t count) noexcept
		{
			utf8_buffered_reader_error_check({});

			TOML_ASSERT_ASSUME(history_.count);
			TOML_ASSERT_ASSUME(negative_offset_ + count <= history_.count);

			negative_offset_ += count;

			return negative_offset_
					 ? history_.buffer + ((history_.first + history_.count - negative_offset_) % history_buffer_size)
					 : head_;
		}

		TOML_NODISCARD
		bool peek_eof() const noexcept(!TOML_COMPILER_HAS_EXCEPTIONS)