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

sql_gen.py: ensure_not_null(): If input column cannot be ensured to be NULL, pass any raised exception through rather than suppressing it and leaving the column in a nullable state

schemas/functions.sql: _merge: Changed indexes to use `COALESCE` to match what sql_gen now does

schemas/functions.sql: _alt: Changed indexes to use `COALESCE` to match what sql_gen now does

sql_gen.py: CompareCond.to_str(): Handle nullable columns using ensure_not_null()

sql_gen.py: ensure_not_null(): Raise NoUnderlyingTableException if can't ensure not null for that reason

sql_gen.py: is_underlying_table(): Support non-Table inputs