Refactor blueprint parser nodes to an interface

Refactor the blueprint parser Value object, which contained a Type enum
and members to hold every possible type, into an interface (now called
Expression).  Rename the existing Expression object that represented a binary
operator Operator.

Also adds and fixes some new printer test cases with mulitline expressions.

Change-Id: Icf4a20f92c8c2a27f18df8ca515a9d7f282ff133