Add -fwrapv to gcc/clang flags.

We rely on signed overflow in many places (in particular for *detecting*
signed overflow), so we need to tell GCC and Clang to not perform any
optimizations that rely on it being undefined.
Remove now-unused dyn_array_ptr_set () function.
Add/update some .gitignore files so that a clean build + `make check` results in a clean repo.
Silence a gcc warning.
Add an assert to sgen-hash-table.c!rehash_if_necessary () to make GCC at -O3 shut up.
Convert asserts in sgen-hash-table.c to SGEN_ASSERT ().