Bugfix/issue 1177 keyword var

#### Summary:

Fix parser so that identifiers can have prefixes matching keywords.  This should've taken care of all of the keywords.  

#### Intended Effect:

See above.  The code makes sure there is no identifier character immediately following a keyword (there must be either a space or a non-identifier symbol such as "{").

#### How to Verify:

There are new test models, none of which parsed in Stan 2.5 and they all parse now.

#### Side Effects:

None.

#### Documentation:

The doc was correct already.

#### Reviewer Suggestions:

Anyone.