Allow to use the STRING as the FROM expression

The sample SQL:
```sql
with "abc" as (select 1 as a) select * from "abc"
```

But got:
```
    SQL Parse Failed: line 0:45 expected table name or subquery, 
    got &{45 48 <string> abc 0 false} with "abc" as (select 1 as a) select * from "abc" ^^^
```

refer #37 