bigint: cfg(target_arch = ...) => cfg(target_word_size = ...)
bigint: remove unnecessary method implements
bigint: inlining small functions
bigint: un-ignore test_shr
bigint: Add benchmarks
bigint: fix wrong benchmark fn name
auto merge of #8749 : gifnksm/rust/bigint-cfg, r=pnkfelix

This also fixes #4393.