bufio: new Scanner interface

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

src/pkg/strings/strings.go

@@ -305,7 +305,8 @@ func SplitAfter(s, sep string) []string {
 }
 
 // Fields splits the string s around each instance of one or more consecutive white space
-// characters, returning an array of substrings of s or an empty list if s contains only white space.
+// characters, as defined by unicode.IsSpace, returning an array of substrings of s or an
+// empty list if s contains only white space.
 func Fields(s string) []string {
 	return FieldsFunc(s, unicode.IsSpace)
 }