What is the equivalent of SQL Server's db_datareader/db_datawriter in PostgreSQL?
There is no builtin role like that in PostgreSQL.  However, you can set up one:

CREATE ROLE db_datawriter;


After this, you have to assign write privileges (usually INSERT, UPDATE, DELETE, maybe TRUNCATE) to it.  To see how to do this for all present and future tables, read my older answer.

Normally, write access also implicates reads, therefore it make sense to add db_datawriter as a member of db_datareader (which was set up similarly, but with SELECT only):

GRANT db_datareader TO db_datawriter;


Then you can grant these two to different login users (or other roles) as necessary:

CREATE USER example_user IN ROLE db_datareader;  


Note that CREATE USER implicates being able to log in, unlike CREATE ROLE.