No documentation on byte layout or formatting

[ShadowJonathan]

Partially re: #17 (comment), I'm currently coming across the problem of seeing all {to,from,as}_bytes functions documented with nothing more but stubs.

I do, for example, not know if the formatting of those bytes are specific to ed25519-dalek, if they're generic DER formats, or if it is any other kind of format, I do not know what I can feed it to give me a valid result, and figuring this out requires me to annotate my own code as to what the functions from this library are actually doing.

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Also, the documentation here comes over as unnecessarily hostile to the user of this library;

Absolutely no validation is done on the key. If you give this function bytes which do not represent a valid point, or which do not represent corresponding parts of the key, then your Keypair will be broken and it will be your fault.