Future: define `fail` and disallow `raise`
create bare biocaml_async library
Lwt_internal_pervasives: don't include Lwt

Remove Lwt in biocaml_lwt for consistency with biocaml_async. Idea was
that doing `open Biocaml_lwt_internal_pervasives` would also open Lwt,
but the analogous thing cannot be done for Async due to module name
clashes. Stream is in both Biocaml_internal_pervasives and Async.Std.
It could of course be done by explicitly listing at Async’s modules as
we do for Core in Biocaml_internal_pervasives, but I’m being lazy.
define Future_async module
Biocaml_async: define Fastq module