Features from optional dependencies

heapless implicit feature

Enables heapless

heapless:

static friendly data structures that don't require dynamic memory allocation

Affects rw-exact-ext::ReadExactExt.read_heapless_vec_exact

num-traits implicit feature

Enables num-traits

num-traits:

Numeric traits for generic mathematics

Affects rw-exact-ext::ReadExactExt.read_num_be, rw-exact-ext::ReadExactExt.read_num_le, rw-exact-ext::ReadExactExt.read_num_ne, rw-exact-ext::WriteAllExt.write_num_be, rw-exact-ext::WriteAllExt.write_num_le, rw-exact-ext::WriteAllExt.write_num_ne