AstGen: add compile error for decl name conflicts

 * 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.

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