[ntuple] remove move-only restriction on RFieldDescriptor
[ntuple] remove move-only restriction on RFieldDescriptor

Field descriptors only hold plain data, so allowing copies doesn't break
correctness.

As part of a merge operation, we will need an easy way to make
(near-)duplicate field descriptors. An earlier version of this commit
implemented Clone for field descriptors, but that approach was abandoned
because of verbosity and difficulty implementing an ergonomic builder pattern
for fields.

Fundamentally, field descriptors are not resources, only information
about resources. Each member is cheaply copyable, especially considering
Return Value Optimization.

root-mirror/root,root-mirror/root,root-mirror/root,root-mirror/root,olifre/root,karies/root,karies/root,karies/root,olifre/root,karies/root,root-mirror/root,olifre/root,root-mirror/root,karies/root,olifre/root,olifre/root,root-mirror/root,karies/root,olifre/root,root-mirror/root,root-mirror/root,karies/root,olifre/root,olifre/root,olifre/root,karies/root,karies/root,olifre/root,karies/root,karies/root,root-mirror/root,olifre/root,root-mirror/root