Nothrow std.conv.to with explicit default value

[Per Nordlöw]

On Monday, 18 June 2018 at 21:10:03 UTC, Steven Schveighoffer 
wrote:
> It just means re-doing std.conv.to, which is pretty hairy, but 
> also pretty well-organized.

Ok. Where in std.conv do the string-to-enum conversions take 
place?