Use iota generator for constant values.
use unexported const names.
cfg: export considerably less.
cfg: rename mkNode to makeNode.

as per go conventions.
cfg: use 'makeNode' consistently.
cfg: document that go_graph kills the C-based CFG.
cfg: defer the C.free's to follow idiom.
cfg: formatting of retvals

no need for parens when there's just one.
cfg: delete identify_loops.

the depth calculation gives us not only whether a node is inside a
loop, but how many loops that node is enclosed within.  so this
isn't really relevant anymore, and is insufficient for what we
need, anyway.
cfg: deleting distance calculation.

there's no reasonable analysis that would care about distance; we
care about more abstract properties, such as reachability.
cfg: more changes that e caught, minor ones in this commit.

mostly naming and clarity issues.