linux: improve backtrace bindings
linux: improve backtrace bindings

Some minor improvements have been made to backtrace_symbols; we no
longer leak the return value and it is marked as nullable. This also
adds a Backtrace namespace for some better bindings which treat the
backtrace as an array of void* instead of just a void*.

frida/vala,flobrosch/vala-fixes,frida/vala,frida/vala,rvu95/vala,spidermonkdust/vala,frida/vala,GNOME/vala,spidermonkdust/vala,GNOME/vala,GNOME/vala,flobrosch/vala-fixes,GNOME/vala,spidermonkdust/vala,flobrosch/vala-fixes,rvu95/vala,rvu95/vala,frida/vala