Handle variables in Operations

[stevan]

The Operation objects do not know how to handle variable definitions at all.

The spec is here.

An operation can have a variable definition, which is populated through either default values or the variables feature of the Execution phase. This variable is then in scope for the query, which means we also have variable accessing. Here is a rough breakdown of the tasks involved.

• Variable definitions in Operations
• Variable resolution within Operation body
• Variable input during exection