Add SingletonContext.Eval to evaluate ninja strings

Our use case is to write out some configuration variables to give to our
legacy build system. But in order to do that, we either need to keep a
Go copy of all of the configuration, and still have all the calls to
convert them to ninja variables, or evaluate our ninja variables.

Change-Id: If9dda305ed41bc8aabe57dd750a74d8b9af1d1a4