Allow optional *args with base AnsiFunction

Allow optional *args with base AnsiFunction

Amended the :class:`.AnsiFunction` class, the base of common SQL
functions like ``CURRENT_TIMESTAMP``, to accept positional arguments
like a regular ad-hoc function.  This to suit the case that many of
these functions on specific backends accept arguments such as
"fractional seconds" precision and such.  If the function is created
with arguments, it renders the the parenthesis and the arguments.  If
no arguents are present, the compiler generates the non-parenthesized form.

Fixes: #4386
Change-Id: Ic492ef177e4987cec99ec4d95f55292be8daa087