[Issue 20889] New: Support construction of std.bigint.BigInt from a sign and a byte-array magnitude

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20889

          Issue ID: 20889
           Summary: Support construction of std.bigint.BigInt from a sign
                    and a byte-array magnitude
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: phobos
          Assignee: nobody at puremagic.com
          Reporter: n8sh.secondary at hotmail.com

Support construction of std.bigint.BigInt from a sign and a byte-array
magnitude. Having it in the library would avoid the necessity of workarounds
seen in the wild like
https://github.com/shove70/crypto/blob/4052a5ef41c604b274ffff8bf6011180cd3a474d/src/crypto/bigint.d#L91-L112

See related https://issues.dlang.org/show_bug.cgi?id=13804

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