* Fix -Wsign-conversion in bigint::subtract_aligned.
n is assigned a size_t, and only used for comparisons with j.
j is assigned 0, compared to n (size_t), and passed to basic_memory_buffer::operator[] (size_t).
* Fix -Wsign-conversion in bigint::assign.
num_bigits is initialised to 0, is only ever incremented, and is passed to basic_memory_buffer::operator[] (size_t) and basic_memory_buffer::resize (size_t).
refnum