Fixed a bug in the semantic analyzer related to `?:` and structures

Thanks to the StackExchange user kaya3, I discovered and fixed two bugs in my compiler (one in the core of the compiler and one in the semantic analyzer), both related to the ternary conditional operator ?: and structures. You can read more about it on this GitHub issue. Also, I've improved the documentation (most importantly, adding an example program in both dialects of AEC).