This PR changes the way that we deal with duplicate table aliases in the binder. Previously, we would throw an exception when a duplicate binding was registered - which is the same behavior that Postgres has, i.e.:

select * from (select 42) t, (select 84) t;
-- Binder Error: Duplicate alias "t" in query!

This is not strictly necessary - however. A duplicate alias is only a problem if there is a conflict. For example, the following query has no ambiguity, and hence can be safely executed (and SQLite can execute it):

select count(*) from (select 42) t, (select 84) t;

After this PR - we can execute this query. Instead, an error is thrown when there is ambiguity when binding a column. For example, this throws an error:

select t.x from (select 42 x) t, (select 84 x) t;
-- Binder Error: Ambiguous reference to table "t" (duplicate alias "t", explicitly alias one of the tables using "AS my_alias")

But this now works, since there is no ambiguity:

D select t.x from (select 42 x) t, (select 84 y) t;
┌───────┐
│   x   │
│ int32 │
├───────┤
│    42 │
└───────┘