Implemented AST for regular commands

[Yurand2000]

The module mechanism with the generic annotation type is fitting for the task, but a number of issues are present:

• Names are short and can be confusing, I would suggest having longer names (as an example, make 'rcmd' into 'RegularCommand')
• Both the @@ and @! operators are confusing, prefer functions with names.
• All the constructors for the various expressions and commands belong to the same module (see test_ast_program.ml at the time of writing), while it would be better to distinguish which base type they come from. I know that types are not being mixed with the original approach, but this is always for the sake of readability.

Other comments:

• Is it possible to implement a print function which also prints the opaque annotation type?
• Is it possible to have the ASTRegularCommands module inside the root module instead of it being into Ast_program?

[Alex23087]

Addressing these issues in order:

• Agree on the naming, will be changing them exchanging conciseness for clarity;

• Same as previous point;

• That is a good point, working on it.

• In general it is not possible, as the annotation is not necessarily a Showable type. A possible design choice is defining a default print function that behaves like [@opaque], and allowing the user to pass a different one. Making this work together with ppx could make the code and the instantiation of the module more clunky;

• Not sure about that, if OCAML defines a module for each file, probably no.