std.conv: to and parse dont work correct with negative numbers

[dlangBugzillaToGithub]

bugzilla (@berni44) reported this on 2019-12-16T16:23:56Z

Transfered from https://issues.dlang.org/show_bug.cgi?id=20452

Description

When the radix is not 10 the conversion from and to int does produce wrong results for negative numbers

---
import std.stdio;
import std.conv;

void main()
{
    foreach (i;2..17)
        writeln(to!string(-1, i));
}
---

produces

---
11111111111111111111111111111111
102002022201221111210
3333333333333333
32244002423140
1550104015503
211301422353
37777777777
12068657453
-1
1904440553
9BA461593
535A79888
2CA5B7463
1A20DCD80
FFFFFFFF
---

The same for the conversion from string to int:

---
import std.stdio;
import std.conv;

void main()
{
    writeln(to!int("-1", 10)); // works
    writeln(to!int("-1", 11)); // ConvException
}
---