* Remove the ability for GenZir parent Scope to be null. Now there is a
Top Scope at the top.
* Introduce Scope.Namespace to contain a table of decl names in order
to emit a compile error for name conflicts.
* Fix use of invalid memory when reporting compile errors by
duplicating decl names into a temporary heap allocated buffer.
* Fix memory leak in while and for loops, not cleaning up their
labeled_breaks and store_to_block_ptr_list arrays.
* Fix stage2 test cases because now the source location of redundant
comptime keyword compile errors is improved.
* Implement compile error for local variable shadowing declaration.
8344a50e1c