Added alter-table support to Producer::SQLServer

Added alter-table support to Producer::SQLServer

I refactored most of the existing code into a method 'build_field_clause'.
I added some basic support for translation from MySQL blob types.
I added some logic to calculate appropriate blob size parameters.
I wrote new 'alter' methods for add/drop of indexes and constraints.
I rearranged the utility function for renaming constraints so that it is a separate operation from quoting.
All identifiers are quoted by default, now.

I can't vouch for completeness or correctness, but it works for quite a few more cases than previously.