The new implementation is backwards incompatible, but I couldn't find
any usage outside mruby and I also couldn't think of a different and
good name.

All ISO C99 printf conversion specifiers for floating point numbers and
an optional precision are supported.

It is largely based on code from the MIT licensed musl libc
(http://www.musl-libc.org/) and its floating point printing is exact
(unlike the current code behind Float#to_s).