sim: Refactor how serialization types are handled in the backend.

sim: Refactor how serialization types are handled in the backend.

The parseParam and showParam functions partially worked using template
specialization, and partially worked using function overloading. The
template specialization could be resolved later once other functions
were added, but the regular function overloads could not. That meant
that it was practically impossible to add new definitions of those two
functions local to the types they worked with.

Also, because C++ does not allow partial specialization of template
functions, it would not be possible to truly use specialization to wire
in BitUnion types.

To fix these problems, these functions have been turned into structs
which wrap static functions. These can be partially specialized as
desired, making them compatible with BitUnions. Also, it's not possible
to overload structures like it is with functions, so only specialization
is considered, not overloading.

While making these changes, these functions (now structs) were also
reworked so that they share implementation more, and are generally
more streamlined.

Given the fact that the previous parseParam and showParam functions
could not actually be expanded beyond serialize.hh, and were not
actually called directly by any code outside of that file, they should
have never been considered part of the API.

Now that these structs actually *can* be specialized outside of this
file, they should be considered part of the interface.

Change-Id: Ic8e677b97fda8378ee1da1f3cf6001e02783fde3
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/36280
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Richard Cooper <richard.cooper@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>