now that we are using C++20, it'd be more convenient if we can use
the <=> operator for comparing. the compiler creates the 6 other
operators for us if the <=> operator is defined. so the code is more
compacted.
in this change, `big_decimal::compare()` is replaced with `operator<=>`,
and its caller is updated accordingly.
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>