Uncompress R data files from the loaded buffer instead of re-reading

Instead of `seek`-ing back to the beginning of the RData/RDS file when
I detect the magic marker for their being compressed, I can just have
IO::Uncompress work directly on the buffer into which the data was
loaded. This a) avoids accessing the file twice; and b) eliminates the
need to use `seek`. Not using `seek` is particularly important because
I can't call `$fh->seek` on Perl 5.10 when $fh is a file handle, and
because, strictly speaking, the constraint for $fh is that it be a
file handle, and not necessarily seekable.
Change the minimum Perl version from 5.12 to 5.10

Following cb6d0e, nothing in the package code requires features not
available in Perl 5.10.

Note that currently some of Dist::Zilla prerequisites do not work
5.10, so authoring the distribution will still need a newer Perl.
Release version 0.91

Changes from v0.8:

- Lower the minimum Perl version to 5.010
- Eliminate 'seek' on RDS/RData file handles

NOTE: v0.9 was an accidental release that only changed the version
number.