compiler: literal handling

[tsatke]

See #55

For the AST, I'd like two new interfaces and three new structs.

type (
    ColumnReferenceExpression interface { ... }
    LiteralExpression         interface { ... }

    ConstantLiteral struct {}                  // implements LiteralExpression
    ConstantLiteralOrColumnReference struct {} // implements both LiteralExpression and ColumnReferenceExpression
    ColumnReference struct {}                  // implements ColumnReferenceExpression
)

I expect that this facilitates handling in the engine a lot.
Whoever implements this; feel free to change the names suggested above. However, the names should be somewhat consistent with what we already have.