- support for cdecimal

- support for cdecimal
- add --with-cdecimal flag to tests, monkeypatches cdecimal in
- fix mssql/pyodbc.py to not use private '_int' accessor in decimal conversion
routines
- pyodbc version 2.1.8 is needed for cdecimal in any case as
previous versions also called '_int', 2.1.8 adds the same string
logic as our own dialect, so that logic is skipped for modern
pyodbc version
- make the imports for "Decimal" consistent across the whole lib.  not sure
yet how we should be importing "Decimal" or what the best way forward
is that would allow a clean user-invoked swap of cdecimal; for now,
added docs suggesting a global monkeypatch - the two decimal libs
are not compatible with each other so any chance of mixing produces
serious issues.  adding adapters to DBAPIs tedious and adds in-python
overhead.  suggestions welcome on how we should be doing
Decimal/cdecimal.