Replace read_rda() by FileIO integration

[alyst]

read_rda() calls is replaced by the generic FileIO load() mechanism as proposed in #2.

• Do we need any other rdata loading methods?
• The tests pass, but I'm not sure whether detect_rdata() is actually called. fixed
• Should the user explicitly type using FileIO or RData should handle this (how?)? FileIO.load is exported

[ararslan]

Since you have FileIO in REQUIRE and in a using statement, I think you can just prefix the FileIO functions when you overload them and it should just work, but I'm not really sure about that.

[alyst]

@ararslan After briefly looking into FileIO I think that it's not done by overloading. Rather, FileIO explicitly calls Library.load() method, where the Library is a package that provides support for a given file format. But magic- or filename-based dispatching is done by load(filename::String) method declared in FileIO. So upon loading RData one would also need to export FileIO.load.
Is that what @reexport macro is for?

[ararslan]

import FileIO: load then include load in the list of exports? I think that should be sufficient.

Using @reexport seems like overkill here since we don't need to expose all of the symbols from FileIO to the user, just load.

[alyst]

@ararslan You're right. Added to PR.

So that should be it: read_rda(...) could be deprecated in favour of load(...) + using RData.