Move postgres specfiic code into postgres dialect

[andygrove]

The recent PR for postgres support has led to postgres specific parsing and writing code creeping into the base sqlparser module. We need to move the postgres specific code into the postgres dialect module.

[nickolay]

Hey @andygrove, until #7 is resolved would you be OK with accepting additional dialect-specific code in the generic parser? I'm asking because @benesch has just submitted a PR to support CREATE MATERIALIZED VIEW (nickolay#1) against my fork of sqlparser-rs. Since I intend to get my fork merged into the mainline, I thought I'd ask now rather than later...