v1.3.21 accessor efficiency upgrade

phyloseq 1.3.21 - accessor efficiency
===========
 tiny speed improvements to phyloseq(), prune_taxa(), prune_samples()
and accessors
For instance, ntaxa() and taxa_names() are very slow on large dataset
Some of the highly inefficient approaches are now replaced.
The strategy is described further in the related GitHub issue:
https://github.com/joey711/phyloseq/issues/183