Add some missing function overrides for MsSqlCe

A bunch of tests fails due to missing function overrides in MsSqlCe dialect.

Those functions have supporting counterpart in MsSqlCe, corresponding of the overrides from MsSql2000. Adding them should be easier than ignoring all those tests, especially for current_timestamp.

https://technet.microsoft.com/en-us/library/ms174077(v=sql.105).aspx
https://technet.microsoft.com/en-us/library/ms173482(v=sql.105).aspx

• bit_length (datalength)

• extract (datepart)

• current_timestamp (getdate)

• mod (%)

• iif (case when)