Bugfix/issue 723 functions in 23

#### Summary:

Fixes three outstanding bugs reported for functions in Stan 2.3 as reported in issue #723 and #715 

#### Intended Effect:

* symbol table for function signatures reset at start of each parse
* single int parameter functions no longer generate empty template parameter list
* fixed assignment of double to var by updating assignment in math::matrix, which was missing a template parameter for generality of assignment

#### How to Verify:

There are tests for:

* reparsing the same model in the parser test
* compiling single int parameter model in parser test (and hopefully it'll get compiled at syntax level)
* the assignment, and the functions should each parse now --- there are unit tests for the assignment

#### Side Effects:

None.  In fact, this removes the side effect of a persistent function signature symbol table.

#### Documentation:

These fixes bring behavior up to match the doc.

#### Reviewer Suggestions:

Anyone.