Improve error messages and generate operator<< for records

[danyowdee]

This PR adds support for the following djinni construct:

enum Currency {
    USD;
    EUR;
};

Product = record {
    name: string;
    identifier: string;
    price: i64;
    currency: Currency;
} deriving (show)

ShoppingCart = record {
    items: list<Product>;
} deriving (show)

So you can write the following C++ code:

Product p{"Little Red Corvette", "v0KpfrJE4zw", 1983, Currency::USD};
Product q{"Breakfast Can Wait", "CzWX1gv6u2s", 2013, Currency::EUR};
ShoppingCart c{{p, q}};
std::cout << c << std::endl; /* prints
"""ShoppingCart{ items:[
Product{ name:Little Red Corvette, identifier:v0KpfrJE4zw, price:1983, currency:Currency::USD },
Product{ name:Breakfast Can Wait, identifier:CzWX1gv6u2s, price:2013, currency:Currency::EUR },
] }"""

If Amount would be missing the declaration of conformance to show, the error message from running Djinni would clearly say so:

Record 'Amount' not deriving required operation(s): 'show'