iceberg: add the bench_iceberg benchmark binary

Port src/bench_iceberg.c, the standalone benchmark binary the
module's bench_impl.h is written for (this repo's bench harness has
no per-module include pattern for it, so the source tree's own wiring
is mirrored instead): noinst_PROGRAMS under USE_BENCHMARK +
ENABLE_MODULE_ICEBERG in Makefile.am, a bench_iceberg target in
src/CMakeLists.txt, and a .gitignore entry. The benchmark covers the
group configurations 2-of-3, 3-of-5, 4-of-7, 5-of-9 and 5-of-10.

Verified: ./bench_iceberg builds and runs under both build systems.

.gitignore

@@ -5,6 +5,7 @@ bench_generator
 bench_rangeproof
 bench_internal
 bench_whitelist
+bench_iceberg
 noverify_tests
 tests
 exhaustive_tests