Remove caching of buffers

[samoht]

We are now caching the IO instances instead. Also, this could lead
to tricky bugs when we start to rename files.

This is similar to mirage/index@f8fdc93

[samoht]

Travis fails with:

#=== ERROR while compiling ppx_irmin.2.2.0 ====================================#
# context              2.0.7 | linux/x86_64 | ocaml-base-compiler.4.09.1 | pinned(file:///repo)
# path                 ~/.opam/4.09/.opam-switch/build/ppx_irmin.2.2.0
# command              ~/.opam/4.09/bin/dune build -p ppx_irmin -j 71
# exit-code            1
# env-file             ~/.opam/log/ppx_irmin-5582-e7966f.env
# output-file          ~/.opam/log/ppx_irmin-5582-e7966f.out
### output ###
# File "src/ppx_irmin/deriver/dune", line 4, characters 33-37:
# 4 |  (libraries ppx_irmin_lib ppxlib base))
#                                      ^^^^
# Error: Library "base" not found.
# Hint: try: dune external-lib-deps --missing -p ppx_irmin -j 71 @install

which is a sensible error but unrelated to that PR so merging.

[craigfe]

I'll make a PR to fix that.