PDL: Remove Located and Named trait

The traits were not used for dynamic dispatch nor
for generic code.
The `Named` trait was also directly implemented on
stdlib types (via FieldPath).

As the trait was not giving any advantages over
a direct impl, remove them and put methods
directly on the ast nodes.

Test: compiles
Change-Id: I5ccf498cf985ea4206c2fb42bed5e0d7eb3c5b92