Make Variables be independent Statements

[haikalpribadi]

At the moment, whenever we want to provide a variable into a Statement builder, we must construct a full Statement object that only contains that one Variable. For example:

type("marriage").relates(var("x"))

In the case above, Graql.var("x") returns a Statement object, in which StatementTypeBuilder.relates() accepts.

This is problematic for various reasons:

• Wherever a query builder expects a variable, the user can pass a Statement with more information leaked into the query, which may not be semantically correct
• We cannot differentiate different scenarios of query building, for example:
  • A concept type has a type followed by a default value: type("person").has("age", 0)
  • A concept type has a var cannot be followed by anything: type("person").has(var("x"))

[flyingsilverfin]

The "tightness" of the APIs was resolved in the refactor for version 2.x