Community Wiki answer generated from question comments by Justin Cave
You can use
dba_constraints) to iterate through all the indexes and constraints that you own/ have access to/ exist in the database.
I can't imagine that you'd really want to try to disable every constraint in the database. Maybe all those in your schema. More likely, you want to disable those that are on a particular set of tables. As you iterate through them, you could construct the appropriate DDL statement to do whatever you want.
You can't disable an index (well, other than a function-based index). You could make the index unusable but that's unlikely to be what you want. You can drop the index but that presumably implies that you're going to save off the definition of the index so you can re-create it.