Replace core language HexStrings with library entity

[bearophile via Digitalmars-d]

Walter Bright:

> Unfortunately, it needs to be a dropin replacement for x"...", 
> which returns a string/wstring/dstring.

This is bad. 99% of the times you don't want a 
string/wstring/dstring out of a hex string:

https://issues.dlang.org/show_bug.cgi?id=10454
https://issues.dlang.org/show_bug.cgi?id=5909

Bye,
bearophile