Overhaul for preparing an option for more compact codegen

- Allow specification of the type for holding binary exponent in ieee754_binary_traits
- Remove some unneeded stuffs / code cleanup
- Fix ieee754_binary_traits::is_nonzero; it worked only when carrier_uint has exactly total_bits-many bits
- Add tiered log computation scheme
- Allow specification of the type for holding decimal exponent in decimal_fp
- Parameter type for get_cache is now template
- on_trailing_zeros/no_trailing_zeros now take Format rather than FormatTraits
- New policy-kind "preferred integer types" is added (supposed to provide the type for decimal exponent as well as the remainder for the first division, but not yet implemented properly)