Add read support for some missing raw parse nodes
The node types A_Const, Constraint, and A_Expr had custom output
functions, but no read functions were implemented so far.
The A_Expr output format had to be tweaked a bit to make it easier to
parse.
Be a bit more cautious about applying strncmp to unterminated strings.
Also error out if an unrecognized enum value is found in each case,
instead of just printing a placeholder value. That was maybe ok for
debugging but won't work if we want to have robust round-tripping.
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/
4159834.
1657405226@sss.pgh.pa.us