Improve compatibility

(1) Replace std::uintXX_t by std::uint_leastXX_t, apply UINTXX_C when needed, and add XX-bit masking if necessary, removing the assumption of integers being XX-bits (but didn't touch the table entries; should fix the table generator for that)
(2) Apply the change discussed in #57
(3) Remove remaining C++14/17 feature usages from to_chars.
(4) Simplify dragonbox_to_chars.cpp

Author: jk-jeon