Implement library exceptions, fix decoding logic.

[toots]

Okay, admittedly this got a little out of hands but eventually I believe that these are great changes. Here's the rundown:

• Use library-specific errors, wrap previous Failure exception into one of these
• Since logs can potentially be OCaml callbacks, we have to make sure that each call to a library function release the global lock. This includes freeing function (except for those inside the GC finalizers.. and we have to hope they don't log..). A couple do not (checked the source) but also I could have missed some.
• Thus, cleanup the general logic and raise as soon as possible.
• Finally, fix decoding logic, same as in #29

I also changed the semantic a bit to follow the underlying library's. Typically, raise Eof instead of having a specific return type. This makes the C code much more simple and easy to follow..

Thanks to the awesome test suite, I believe that the PR is ready for review now. Will be testing it locally as well.

[toots]

@gndl, let me know how you feel about these changes. I think that it makes the binding better and leaner. I can try to clean them up more if you want.

[gndl]

The new exception format is a big improvement. I am less convinced by the immediate raising of exceptions (risk of memory leaks among others) but if you prefer this style, I do not object. I am therefore in favor of this changes.

[toots]

Great, thank you. I'll have another pass to double check about memory leaks, you are totally right about this.

[toots]

Just had another pass at it, this looks good to me now.

[smimram]

Please also adapt the code in Liquidsoap!

[toots]

Please also adapt the code in Liquidsoap!

It's coming!

[toots]

@gndl I'm gonna release this. There might still be bugs but I'll be ready to fix them asap and re-release. This will help smoothing the process for liquidsoap next release.