Render N'' for SQL Server unicode literals

Render N'' for SQL Server unicode literals

The ``literal_processor`` for the :class:`.Unicode` and
:class:`.UnicodeText` datatypes now render an ``N`` character in front of
the literal string expression as required by SQL Server for Unicode string
values rendered in SQL expressions.

Note that this adds full unicode characters to the standard test suite,
which means we also need to bump MySQL provisioning up to utf8mb4.
Modern installs do not seem to be reproducing the 1271 issue locally,
if it reproduces in CI it would be better for us to skip those ORM-centric
tests for MySQL.

Also remove unused _StringType from SQL Server dialect

Fixes: #4442
Change-Id: Id55817b3e8a2d81ddc8b7b27f85e3f1dcc1cea7e