If c_marshaller is provided during g_signal_new() registration, the
automatic va_marshaller will not be set. If we leave the c_marshaller as
NULL in the simple cases, both a c_marshaller and va_marshaller will be
set for us.

This is particularly helpful when dealing with stack traces from Linux
perf, which often cannot unwind the stack beyond the ffi_call_unix64
stack-frame on x86_64.

Related to Initiatives#10