sanity_check methods throughout the codebase shouldn't be part of the API

[apoelstra]

Actually I'll just file an issue. There are a lot of sanity_check methods in the code, many of which are not called from within the library, and at least one has a doc example suggesting to use it to check "whether all spend paths are accessible in the Bitcoin network".

So we need to re-assess all these functions and see if they can be pulled into the type system somehow so you simply can't create the objects without running the (non-pub) methods.

Originally posted by @apoelstra in #652 (comment)