Transfer of sourceR from depending on hashmap to fastmap.

I've swapped out the dependency on the archived 'hashmap' package, and replaced with the 'fastmap' package. Speed appears to be similar, although R/node.R:363 is a bottleneck.

Recommend test and upload to CRAN. N.B. the RNG appears to have changed, so the MCMC tests are now not working. Luckily, they are set to avoid the CRAN test anyway.