Refactor variable

[StrykerKKD]

Variable should be more easily assignable, because right now is a little bit cumbersome. ~~Also the variable needs to be more expressive, so it should be a record instead of a simple string.~~

Variable's module should have constructor for every kind variable type.

Another issue is that you can only get the variable's value by calling State's value_of, which is not problem, but maybe it should live in Variable's module.

Variable module would have:

• type specific constructors which would give back a goal, so it would be just an abstraction over Goal.equal
• value_of function which has a variable_name and a state input and returns the value of the variable and this return value can either be a success or a failure(option and an exception version)

[StrykerKKD]

Adding namespace to a variable also could be included in this issue. Adding namespace would make sense, because:

• It could make the variable more unique
• Namespace could be used easily to map to a configuration's field

I'm not sure about adding namespaces, because it add an unnecessary overhead when the namespace could be part of the variable's name.