This is a fork of the flt2dec portion of rust-strconv [1] with
a necessary relicensing (the original code was licensed CC0-1.0).
Each module is accompanied with large unit tests, integrated
in this commit as coretest::num::flt2dec. This module is added
in order to replace the existing core::fmt::float method.

The forked revision of rust-strconv is from 2015-04-20, with a commit ID
9adf6d3571c6764a6f240a740c823024f70dc1c7.

[1] https://github.com/lifthrasiir/rust-strconv/

As a side effect `core::fmt::float` is gone now. This also slightly
changes the float output, so this is:

[breaking-change]

The bug involves the incorrect logic for `core::num::flt2dec::decoder`.
This makes some numbers in the form of 2^n missing one final digits,
which breaks the bijectivity criterion. The regression tests have been
added, and f32 exhaustive test is rerun to get the updated result.

The master no longer has `std::num::Float`, so a generic `ldexp` is
not readily available. `DecodableFloat::ldexpi` works around this.

For the shortest mode the IEEE 754 decoder already provides
an exact rounding range accounting for banker's rounding,
but it was not the case for the exact mode. This commit alters
the exact mode algorithm for Dragon so that any number ending at
`...x5000...` with even `x` and infinite zeroes will round to
`...x` instead of `...(x+1)` as it was. Grisu is not affected
by this change because this halfway case always results in
the failure for Grisu.