Implement validator module

[Kn99HN]

Validator module represent the parsing of validator language:

Validator {
    Process_name: {
       repeat between 1 and n {

      },
      maybe {

      }
   }
}

[joyceluyy]

I think we first need to parse the expectation to a tree, with each defined expectation statement (such as task, send, receive) as the node.
And create check function for each of the expectation statement to return possible number of events it could match in the trace tree from a given node.
Eventually we can create a validation/checker function that takes in both expectation tree and the trace tree, leverage the check functions to find if there is a possible matching path

[Kn99HN]

I think that's reasonable. The validator module that I define in this issue is only used for parsing from files.