use environments to lazily initialize benchmarks

[kuk0]

if we only need to run a few benchmarks,
e.g. text-benchmarks Pure/drop/Text+tiny
it's wasteful to initialize all of them
(and read all the test data in memory - all
the test data is a few hundreds of MB, but
we read it in several tests, as different types,
and most seriously, as String - which turns this
into GBs of memory)

[kuk0]

in fact, i would like to either split or completely delete the Strings from the benchmarks; just reading 100MB of text as a String is slow and i'm not really convinced the String benchmarks are useful

[hvr]

Thanks!