rstl: Return reference to emplaced element in emplace_back()
rstl: Return reference to emplaced element in emplace_back()

Follows the behavior of the C++17 variant of emplace_back, which returns
a reference to the emplaced element.

This allows eliminating cases like:

container.emplace_back(some, things, arguments);
container.back().blah();

... // subsequent modifications to the element

given we can just retrieve the reference from the emplace_back call.

AxioDL/PathShagged,AxioDL/PathShagged,AxioDL/PathShagged