Add depreciation to B64.decode function

[dinosaure]

No description provided.

[avsm]

I don't think this is quite right -- don't we want to revise the interface so that the decode function returns an option in the future, and decode_exn will return an exception? If we want decode_opt, then there is no need to deprecate decode as the exception is documented.

[hannesm]

I agree with @avsm (at least partially): I'd be in favour of decode returning either an option or a proper Error (containing some information, including at which byte of the input it encountered an error), and don't provide a decode_exn (if this is needed by someone, they can always write it themselves, but in general it is unsafe and does not conform to the MirageOS guidelines "don't leak exceptions")

[dinosaure]

Ok, I will do the update, in my mind is to not break the API but if it's fine, I agree with you.

[avsm]

Closed in favour of #24