Bugfix/issue 866 vars prefix

#### Summary:

Fix bug that prevented statement-initial variables with prefixes matching a Stan type.

#### Intended Effect:

Allow variables to have prefixes matching type names.

#### How to Verify:

There is a new model checked into syntax-only which fails in 2.4 and passes in this branch.

#### Side Effects:

None.

#### Documentation:

This brings behavior up to existing doc.

#### Reviewer Suggestions:

Anyone.