Error message for sequence without a known length

[dtolnay]

bincode can't encode infinite sequences

A couple things:

• Neither can Serde!
• This message is used for maps too, so it should be more general or split into two messages.
• This message doesn't help the user fix the problem. ("Make your sequence less infinite"?)

[dtolnay]

Something like this might work better:

Bincode requires sequence length to be available before serializing any elements

[TyOverby]

Fixed in eddf897