Decimal string to floating point conversion with correct half-to-even rounding

[9il]

On Saturday, 4 July 2020 at 20:35:48 UTC, Walter Bright wrote:
> On 6/21/2020 8:24 AM, 9il wrote:
>> So excited to finally announce we can correctly parse 
>> floating-point numbers according to IEEE round half-to-even 
>> (bankers) rule like in C/C++, Rust, and others.
>
> Great work! Would you like to add it to dmd?

Thank you! Yes.

It would be very much appreciated to preserve the `mir.` 
namespace for the `parse` module, the required `bignum` package*. 
Also, whenever the code will be located I would like to have 
control over it. Will you agree?

* - `mir.bignum` is 6K LOC and it is expected to grow up to 20K 
LOC if finished. The package includes abstract views for big 
integers, decimal, and binary FP numbers; stack-allocated big 
integers; midsize unsigned integers; software FP numbers with 
extended precision.