vapigen: Support overriding memory management functions in metadata
vapigen: Support overriding memory management functions in metadata

Fixes bug 589307.

jessevdk/vala,flobrosch/vala-fixes,severinh/vala,jessevdk/vala,rvu95/vala,flobrosch/vala-fixes,tjol/vala-tjol,kamanashisroy/aroop,playya/vala,phako/vala,xdave/pvala,Jc2k/vala,kamanashisroy/aroop,Distrotech/vala,rvu95/vala,frida/vala,tjol/vala-tjol,themiwi/vala,GNOME/vala,playya/vala,kamanashisroy/aroop,GNOME/vala,spidermonkdust/vala,spidermonkdust/vala,lovebug356/vala,spidermonkdust/vala,severinh/vala,lovebug356/vala,themiwi/vala,playya/vala,flobrosch/vala-fixes,themiwi/vala,frida/vala,tjol/vala-tjol,frida/vala,frida/vala,severinh/vala,phako/vala,GNOME/vala,Distrotech/vala,jessevdk/vala,frida/vala,Distrotech/vala,rvu95/vala,Jc2k/vala,GNOME/vala