Change dwarf to avoid nesting CUs

[graydon]

The source file CUs should not be nested in the crate CU. They should be separate: first the crate-file CU, then the source-file CUs, in sequence. This will probably not break much, possibly the crate reader will need to change a bit in the runtime.

Apparently the nesting is not something other dwarf-speakers expect.

[graydon]

(a) fixed ages ago, (b) obsolete since we're dropping DWARF of this sort