sim: Move paramIn/Out definition to header file

sim: Move paramIn/Out definition to header file

This patch is moving the definitions of paramIn/Out templates to
the header file. In this way we gain:

1) We don't have to do explicit instantiation anymore for user defined
types. This spares us from including data type header files into
serialize.cc

2) We can overload show/parseParam for BitUnions or any other type
that requires special handling when serializing. Just by overloading
the two templates we get all the containers' (list, vector, array..)
serialization for free

2) gtest: With the idea of adding unit tests for Serializable objects,
we can avoid importing serialize.cc and just redefine Serializable
methods in the test source, implementing a Serializable stub

Change-Id: I45a9bb87d5ef886a3668fd477005cd105f612e36
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/13635
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>