Improve the handling of signals by leveraging the `Py_AtExit`
functionality to trigger cleanup when the interpreter exits.
When receiving SIGINT handle it specially because exceptions in python
are only thrown when the interpreter is running and not when C code is.
When initializing `revng::InitRevng` it is assumed by its parent,
`InitLLVM`, that the lifetime of `Argv` and be equivalent to
static, because it might be later retrieved to print it in case of a
crash. This works with the traditional `main`, however in the pipebox
the string passed are not guaranteed to have that lifetime. This commits
moves ownership of the strings to a class that stores them to avoid this
problem.
The `serialize` function of `revng::pypeline::Model` erroneously
returned a buffer instead of `bytes`, fix the function so that `bytes`
are returned instead.
Giacomo Vercesi