literal handling

[tsatke]

We must differentiate between a constant literal and a column reference in a projection.

The following changes should be implemented.
There is a difference between

1. SELECT name
2. SELECT "name"
3. SELECT 'name'

• For 1, name is a column reference, and must be resolved to a column of the input of the projection. If no such column exists, an error is returned.
• For 2, "name" is a constant literal or column reference, meaning that if there exists a column name in the input of the projection, it is resolved to that column. If not, it is interpreted as a constant literal.
• For 3, 'name' is a constant literal, which in this case is of type String.

TODO:

• adapt parser ( parser: literal handling #56 )
• adapt parse tree ( parser: literal handling #56 )
• adapt AST ( compiler: literal handling #57 )
• adapt compiler ( compiler: literal handling #57 )
• adapt engine ( engine: literal handling #58 )