avoid using impl in error_printer example

[N-Dekker]

What does this change do?

Replaced access to toml::impl::control_char_escapes with manual handling of control characters, in print_string.

Would ease a possible future replacement of #include <toml++/toml.hpp> with import tomplplusplus (because toml::impl is not exported).

Is it related to an existing bug report or feature request?

• It's another follow-up to pull request let error_printer example print source line of parse error #259.

• I've read CONTRIBUTING.md
• I've rebased my changes against the current HEAD of origin/master (if necessary)
• I've added new test cases to verify my change
• I've regenerated toml.hpp (how-to)
• I've updated any affected documentation
• I've rebuilt and run the tests with at least one of:
  • Clang 8 or higher
  • GCC 8 or higher
  • MSVC 19.20 (Visual Studio 2019) or higher
• I've added my name to the list of contributors in README.md

[N-Dekker]

Note: Two simple escape sequences, \a (0x07) and \v (0x0B), are not in toml::impl::control_char_escapes:

tomlplusplus/include/toml++/impl/forward_declarations.hpp

Lines 127 to 142 in fea1d90

	inline constexpr std::string_view control_char_escapes[] =
	{
	"\\u0000"sv,
	"\\u0001"sv,
	"\\u0002"sv,
	"\\u0003"sv,
	"\\u0004"sv,
	"\\u0005"sv,
	"\\u0006"sv,
	"\\u0007"sv,
	"\\b"sv,
	"\\t"sv,
	"\\n"sv,
	"\\u000B"sv,
	"\\f"sv,
	"\\r"sv,

I found them at https://eel.is/c++draft/lex.ccon#nt:simple-escape-sequence-char saying:

simple-escape-sequence-char: one of
  '  "  ?  \ a  b  f  n  r  t  v

Would it be worth adding these two to toml::impl::control_char_escapes as well? Honestly it doesn't matter much to me. Just asking 😃