serialize/unserialize vector improvement

Hi folks,

I've attached a patch to the svn trunk that improves the performance
of the serialize/unserialize interface for vector types. The current
implementation: a) invokes the R_XDREncode operation for each element
of the vector type, and b) uses a switch statement to determine the
stream type for each element of the vector type. I've added
R_XDREncodeVector/R_XDRDecodeVector functions that accept N elements
at a time, and I've reorganized the implementation so that the stream
type is not queried once per element.

In the following microbenchmark (below), I've observed performance
improvements of about x2.4. ?In a real benchmark that is using the
serialization interface to make MPI calls, I see about a 10%
improvement in performance.

Cheers,
--Michael

microbenchmark:

input <- matrix(1:100000000, 10000, 10000)
output <- serialize(input, NULL)
for(i in 1:10) { print(system.time(serialize(input, NULL))) }
for(i in 1:10) { print(system.time(unserialize(output))) }