Validity of get_mut on MaybeUninit

[Dylan-DPC-zz]

Creating this issue to track the discussion on whether this is valid or not, from this FIXME note:

This is calling get_mut on an uninitialized
MaybeUninit (here and elsewhere in this file). Revisit this once
we decided whether that is valid or not.
We can do this only because we are libstd and coupled to the compiler.
(FWIW, using freeze would not be enough; flt2dec::Part is an enum!)

The issue being mentioned when the fixme was closed without addressing this concern, and the new tracking issue doesn't raise it, so created this so that we can discuss it here and link it to the tracking issue

cc @RalfJung

[RalfJung]

Something went wrong and two issues got created, closing in favor of #76092.