Add a new, simple interface for scanning (probably textual) data,
based on a new type called Scanner. It does its own internal buffering,
so should be plausibly efficient even without injecting a bufio.Reader.
The format of the input is defined by a "split function", by default
splitting into lines. Other implemented split functions include single
bytes, single runes, and space-separated words.
Here's the loop to scan stdin as a file of lines:
s := bufio.NewScanner(os.Stdin)
for s.Scan() {
fmt.Printf("%s\n", s.Bytes())
}
if s.Err() != nil {
log.Fatal(s.Err())
}
While we're dealing with spaces, define what space means to strings.Fields.
Fixes#4802.
R=adg, rogpeppe, bradfitz, rsc
CC=golang-dev
https://golang.org/cl/7322088
