Remove Ident nodetype in favor of using String nodes; this fixes some

Remove Ident nodetype in favor of using String nodes; this fixes some
latent wrong-struct-type bugs and makes the coding style more uniform,
since the majority of places working with lists of column names were
already using Strings not Idents.  While at it, remove vestigial
support for Stream node type, and otherwise-unreferenced nodes.h entries
for T_TupleCount and T_BaseNode.
NB: full recompile is recommended due to changes of Node type numbers.
This shouldn't force an initdb though.