Improve monad transformers support

[madidier]

This PR fixes the following limitations:

• Some class instances from the base monad could not pass through SeldaT, ie. SeldaT b (StateT s IO) did not have a MonadState s instance,
• SeldaT could only be used as outermost element in the transformer stack, ie. StateT s (SeldaT b IO) did not have a MonadSelda instance,
• The use of onException made it unsafe to use a base monad that uses any form of short-circuiting that is not an exception, ie. ExceptT.

Fixing that last point requires using generalBracket introduced in exceptions-0.9, so I had to bump the dependency requirement. I am not sure how this impacts the range of GHC versions that can be supported.

Supporting ExceptT/MonadError is rather important for applications based on servant.

[exaexa]

Note for self: we should bump changelog once this is in.

@madidier I think this can go in as is (we'll wait for @valderman for ack). Anyway, it's a year or so, so if there's any new update to add please let us know (but there doesn't seem to be much news in mtl since then so I don't expect much :) ). Thanks!

[madidier]

Hi @exaexa. Thanks for having a look at this PR. To be honest, I haven't touched this project in over a year, so I have no update to share. Should be fine as long as the code still compiles and the tests pass :)

[exaexa]

Should be fine as long as the code still compiles and the tests pass :)

OK thanks for confirm, that indeed seems to be the case :)