Handle identity and function defaults in ADD COLUMN

The majority of this was a little cleanup around the sequence interaction path (1ddc331, now via StoreAdapter instead of QueryContext) and using a custom expression instead of looking up NEXTVAL (df0b810).

In an EXPLAIN, the pseudo-function `_SEQ_NEXT` appears instead of NEXTVAL. My hope was that underscore would appear as special and 'internal'.

While in the area, properly support special function defaults too.