Introduce package level `lvalueInit`

This is a library solution for of my originl "WONTFIX" request:
"Make const(T).init and immutable(T).init lvalues"
https://issues.dlang.org/show_bug.cgi?id=11307

It is currently used in 3 places, but I have plans to add it to 2 other places too (`initializeAll` and a `moveOver` required for emplacing rvalues with disabled postblit).

It also fixes a bug where `move` could not be used with elaborate types that have a disabled default initialization.

Overall, it's just a little convenience that avoids bloat.