Generate .Call(...) instead of .Primitive(.Call)(...) for cppFunction

[krlmlr]

Closes #795.

[eddelbuettel]

This may become my favourite pull request ever:

R> library(Rcpp)
R> cppFunction("double plusTwo(double x) { return x+2; }")
R> plusTwo
function (x) 
.Call(<pointer: 0x7f692cdf0cb0>, x)
R> plusTwo(40)
[1] 42
R> 

We all just looked at each other assuming the other knew why on earth we used .Primitive() in here too.

[eddelbuettel]

Well okay, maybe not as famous as the one by @jjallaire that got us Attributes in the first place ;-)