add dwarf sibling links

[graydon]

Looking something up in dwarf is currently linear in the size of the dwarf
tables. Adding sibling links can drop this to a much faster scan, as we have a
hierarchical module namespace to make use of.

(Worst case we can also add a DIE type for hashed child access or such, but
leave that for future optimization)

[graydon]

WONTFIX (not required for bootstrapping)