This replaces the current implementation of `description` and
`debugDescription` for the standard floating-point types with a new
formatting routine based on a variation of Florian Loitsch' Grisu2
algorithm with changes suggested by Andrysco, Jhala, and Lerner's 2016
paper describing Errol3.

Unlike the earlier code based on `sprintf` with a fixed number of
digits, this version always chooses the optimal number of digits.  As
such, we can now use the exact same output for both `description` and
`debugDescription` (except of course that `debugDescription` provides
full detail for NaNs).

The implementation has been extensively commented; people familiar with
Grisu-style algorithms should find the code easy to understand.

This implementation is:

* Fast.  It uses only fixed-width integer arithmetic and has constant
  memory and time requirements.

* Simple. It is only a little more complex than Loitsch' original
  implementation of Grisu2.  The digit decomposition logic for double is
  less than 300 lines of standard C (half of which is common arithmetic
  support routines).

* Always Accurate. Converting the decimal form back to binary (using an
  accurate algorithm such as Clinger's) will always yield exactly the
  original binary value.  For the IEEE 754 formats, the round-trip will
  produce exactly the same bit pattern in memory.  This is an essential
  requirement for JSON serialization, debugging, and logging.

* Always Short.  This always selects an accurate result with the minimum
  number of decimal digits.  (So that `1.0 / 10.0` will always print
  `0.1`.)

* Always Close.  Among all accurate, short results, this always chooses
  the result that is closest to the exact floating-point value. (In case
  of an exact tie, it rounds the last digit even.)

This resolves SR-106 and related issues that have complained
about the floating-point `description` properties being inexact.

The previous code was unnecessarily obfuscated by the attempt to combine
these two operations.

…ift into tbkka-floating-point-printing-C

Adding a C source file somehow exposed an issue in an unrelated C++ file.
Thanks to Joe Groff for the fix.

Having a C file in stdlib/public/runtime causes strange
build failures on Linux in unrelated C++ files.

As a workaround, rename SwiftDtoa.c to .cpp to see
if that avoids the problems.

This reverts commit 6cd5c20.