Implicit conversion of concatenation result to immutable

[Per Nordlöw]

On Thursday, 1 April 2021 at 21:59:21 UTC, H. S. Teoh wrote:
> Just use .idup on the result.

In the meanwhile `.assumeUnique` from `std.exception` should be 
preferred over `.idup` unless duplicate memory allocations is of 
intrinsic value. ;)