Avoid third seek operation in `Seek::stream_len` when possible
Avoid third seek operation in `Seek::stream_len` when possible

aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust