Mark a constructor of std.format.FormatSpec as @safe, pure
Fix issue 11600: to!BigInt(string) should validate input.
Merge pull request #1578 from tom-tan/safe-pure-nothrow-this-format-FormatSpec

Mark a constructor of std.format.FormatSpec as @safe, pure, nothrow
Merge pull request #1731 from quickfur/issue11600

Fix issue 11600: to!BigInt(string) should validate input.