[common] Unify FFI result types

[danielsn]

          Very likely for a separate PR as well, but it occurred to me that both `ddog_crasht_Result` and `ddog_prof_Profile_Result` are exactly the same `(dummy boolean, ddog_Error)` pair.

Furthermore, they already both use ddog_Error, so I'm thinking it may be relevant to introduce a shared ddog_Result.

Originally posted by @ivoanjo in #551 (comment)