Fatal: The wrappers for exec and system can be reused

Signed-off-by: Niels Thykier <niels@thykier.net>
Rename ScopeUtil to Util and prepare for more utilities

Signed-off-by: Niels Thykier <niels@thykier.net>
Move "fill_protos" to autodie::Util

Signed-off-by: Niels Thykier <niels@thykier.net>
Move "make_core_trampoline" to autodie::Util

Signed-off-by: Niels Thykier <niels@thykier.net>
Move install_subs into autodie::Util

Signed-off-by: Niels Thykier <niels@thykier.net>
Split _one_invocation into an Fatal and an autodie part

This removes the "$back_compat" argument as it is now implies the method
invoked via class inheritance.

Signed-off-by: Niels Thykier <niels@thykier.net>
Move _expand_tag (as expand_import_tag) into autodie::Util

Signed-off-by: Niels Thykier <niels@thykier.net>