elide long containers in pretty

limits sequence length to 1k items

avoids performance issues with pretty-printing large containers
simplify truncated enumeration in pretty

thanks @ivanov!
make max_seq_length an attr on PrettyPrinter

instead of module global
make _enumerate a method on PrettyPrinter

even less duplicate code
allow max_seq_length=0 to disable truncation altogether
Merge pull request #5376 from minrk/pretty-cap

elide long containers in pretty