Add read support for some missing raw parse nodes

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 <[email protected]>
Discussion: https://www.postgresql.org/message-id/flat/4159834.1657405226@sss.pgh.pa.us