Parse record declaration while declaring variable

[leewei05]

Current parser only support type declaration and variable declaration separately. With this update, users can declare both custom type and variable at the same type. Example,

struct animal {
  int lion;
  int tiger;
  int giraffe;
} big_zoo = {
  .lion = 10,
  .tiger = 8,
  .giraffe = 12,
};

Noting that I added a set for tracking custom types, so that the compiler doesn't dump type declaration every time a custom type is used.

[Lai-YT]

COOL 😎

[leewei05]

Added code generation tests since this doesn't require additional code generation implementations.

[Lai-YT]

LGTM!