sql_gen.py: EnsureNotNull: Take a type param instead of a null param so that the EnsureNotNull object stores the underlying column's type

sql_gen.py: underlying_col(): Support non-Col inputs

sql_gen.py: EnsureNotNull: Removed default value for null param to remind user that default value depends on value's type and will not always be a string

sql.py: add_index(): Added ensure_not_null param to disable the ensure_not_null functionality to force a plain index

sql.py: flatten(): Add indexes on the created table so its columns can be used in an O(n) merge join

sql_gen.py: null_sentinels: Added value for integer type

sql_gen.py: CompareCond.to_str(): Always wrap the left-side column if it's nullable. Wrap the right-side value if the left side was wrapped, rather than if both the left and right side are nullable. This causes coalesce() indexes to be used to look up NULL values using the value NULL gets coalesced to, rather than doing a sequential scan.

sql_gen.py: Run truncate() on all identifiers so that literal-string-based lookups for an identifier (such as in db.col_info()) don't use the untruncated value

sql_gen.py: Added truncate()

sql.py: put_table(): Resolving default value column: Fixed bug where default value of None was used as a key for mapping, even though this is an invalid Col name