Bugfix/issue 1151 fun var constraints

#### Summary:

Updated code generator to allow functions to be used in variable constraints in the parameters block.

#### Intended Effect:

See summary.  Pushed `std::ostream*` pointer through the two `transform_inits()` overloads generated for a model.   The functionality has been isolated from the interfaces (RStan, PyStan, CmdStan) through `common/initialize_state.hpp`, which was updated to pass a stream to `transform_inits()`.

#### How to Verify:

There is a new test model in `src/test/test-models/good/fun-var-constraints.stan` that fails to compile in Stan 2.5 but now compiles.

#### Side Effects:

None.

#### Documentation:

Now functionality matches documentation.

#### Reviewer Suggestions:

Anyone.