Fix potential ODR violation

[shewitt-au]

I think there's potential for ODR violations in "lib/include/pl/core/tokens.hpp".

Here's an example:

namespace Keyword {

        Token makeKeyword(const Token::Keyword& keyword, const std::string_view& name) {
            auto token = makeToken(core::Token::Type::Keyword, keyword);
            Token::Keywords()[name] = token;
            return token;
        }

        const auto If           = makeKeyword(Token::Keyword::If, "if"); // <--- Here

Shouldn't this be:

namespace Keyword {

        inline Token makeKeyword(const Token::Keyword& keyword, const std::string_view& name) {
            auto token = makeToken(core::Token::Type::Keyword, keyword);
            Token::Keywords()[name] = token;
            return token;
        }

        inline const auto If           = makeKeyword(Token::Keyword::If, "if");
        ------

I'm new to modern C++. Last time I was a progammer for a living MVSC6 was a thing, so apologies if I'm off the mark. My outdated knowledge is what made this issue, if it is one, stand out.

I waded through standards. It seems a the inline specifier can be applied to variables as well as to functions. This would seem to remove the danger of vioilating the ODR.

I guess it's not an ODR violation in the strict sense if the definitions are identical, but to me it looks ill-formed because it violates the rule requiring only one definition of an object with external linkage — unless it is declared inline.

Sorry if the ODR claim is misleading. I'm not sure how to properly classify this.