[FEATURE] Add structure declaration

[frankhart2018]

Describe the solution you'd like
To add support only struct line has to be parsed as we use a selectively recursive recursive descent parser which does not require unnecessary recursion.

Todo

1. Identify struct as a keyword
2. Parse the line - struct
3. Compile the line - struct

For reference look at implementation of loops and function definition.

[10Kaiser10]

I would like to work on this issue, please assign me.

[frankhart2018]

Sure @10Kaiser10.

[frankhart2018]

Merged.