Simplify SPARQL algebra Projection handling

[abrokenjester]

Related to #2985

For the following query:

select (?foo as ?bar) where { ?foo <foo:bar> "bar" }

We produce the following algebra tree:

Projection
   ProjectionElemList
      ProjectionElem "foo" AS "bar"
   Extension
      ExtensionElem (bar)
         Var (name=foo)
      StatementPattern
         Var (name=foo)
         Var (name=_const_d75d935f_uri, value=foo:bar, anonymous)
         Var (name=_const_17c13_lit_e2eec718_0, value="bar", anonymous)

Note that the aliasing of "foo" to "bar" is represented twice here: once as an ExtensionElem, once as an aliased ProjectionElem.

The ProjectionElem in our algebra predates sparql and its evaluation (in ProjectionIterator) has mechanisms to map source bindings to target bindings that we effectively don't need anymore, since the mapping is handled by the extension already. We should look into simplifying this part of the algebra and remove the distinction between source and target name in the ProjectionElem. This will make the intent of the algebra clearer, and likely allow us to improve performance as well, as the ProjectionIterator can be simplified.

[abrokenjester]

Alternatively we should look into removing the Extension where the projection itself already takes care of the aliasing.

Also as a possible complication is that handling of construct queries currently heavily relies on projection itself supporting aliasing.