Implement testing of to-string implementation via KST

[Mingun]

• Added a new key to KST -- to-string which should contain the string which is expected from to-string implementation.
• Added missed tests for Nim and Rust

Depends on kaitai-io/kaitai_struct_compiler#297 which introduces CppTranslator.doRawStringLiteral method to get a plain old C string literal instead of std::string object.

[generalmimon]

I don't see any benefit in implementing this on the side of the KST translator when we'll eventually need this in KSC anyway - see kaitai-io/kaitai_struct#980 (-webide-representation already has this feature, so if we want to-string to be a visualizer-independent full-fledged replacement for -webide-representation, we're going to need it).

So instead of introducing this unnecessary to-string key to KST specs, it would be better to implement .to_s for structs with to-string on the KSC's side, and then we'll just do actual: struct.to_s or something.

Needless to say that your proposed to-string key only works on the top-level object (there is no way to specify which object we want to convert to string using actual: ...), which is an unnecessary limitation.

[Mingun]

@generalmimon, I'm not sure, how it would be possible to implement in non-ambiguous way.

id: to_string_custom
data: term_strz.bin
asserts:
  - actual: to_s
    expected: '"s1 = foo, s2 = bar"'

would mean that object should have to_s seq field or instance. What the syntax you'll propose for calling to-string?