add ReleaseObject and PreserveObject to protect memory from GC

[randy3k]

It is important when memory of an R object is referenced by a julia variable.

using RCall
x = Reval(Rparse("x = 1:10^8"))
RCall.PreserveObject(x)
y = RCall.rawvector(x);
Reval(Rparse("x = 1"))
for(i in 1:10) 
    Reval(Rparse("gc()"));
end
y

without the RCall.PreserveObject statement, the last y will produce segfault.