transpile: rewrite relooper to be iterative to avoid stack overflow on large CFGs

[Hellboy28D]

Summary

The relooper in c2rust-transpile used a recursive algorithm for control-flow
reconstruction. On functions with very large/deep CFGs (e.g. thousands of
sequential basic blocks, as in the BLAKE-256 example in #1821), this could
exhaust the stack and crash the transpiler.

This PR rewrites the relooper to work iteratively, using an explicit
dominator computation instead of recursion, so structured control-flow
reconstruction no longer depends on stack depth proportional to CFG size.

Changes

• Replaced the recursive relooper implementation in
  c2rust-transpile/src/cfg/relooper.rs with an iterative one
• Added unit tests for the dominator computation

Testing

• cargo test -p c2rust-transpile relooper — all 7 new dominator tests pass
• Transpiled the BLAKE-256 C implementation from Reworked relooper regressed on some large CFGs #1821: output is
  byte-identical to the output produced by upstream master, confirming the
  rewrite preserves translation semantics
• Transpiles previously-failing large-CFG inputs without requiring an
  increased ulimit -s

Notes

Happy to adjust the approach or add more tests based on maintainer feedback.