fixes added, but error in a simple example. doesn't pass yet.
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
very minor. Changed a comment line in phyloseq()

Need to extend the meaningless quotation-mark removal from just the
phylogenetic tree tip names to all taxa_names in all components. It is
fast, even on HMP dataset 45000 OTU names. Not even worth checking for
them first, just run fixed-gsub on all components.
Will need to define a taxa_names()<- assignment S4 generic. Maybe keep
it internal, since you don't really want users assigning name changes
to components willy-nilly
Added some tests that point out a flaw in new prune_taxa methods

Shouldn't be too hard to fix. Think carefully, and check about
re-ordering taxa at pruning step (again). Used to just pass back to
phyloseq constructor.
v1.3.22 updates for

not finished. Need to finish and re-test.
Also need to add tests for new `taxa_names<-` assignment methods
v1.3.22 important infrastructure update

more tests and prune/phyloseq/assign methods.
Now more likely to notice if one causes problems in some of these key
functions/methods. This in-turn makes it easier to make
changes/improvements without wondering about causing a hidden cascading
problem.
Merge pull request #185 from joey711/accessor-efficiency

v1.3.22 infrastructure improvements