This CL introduces the go.Example type and go.Examples functions that
are used to represent and extract code samples from Go source.

They should be of the form:

// Output of this function.
func ExampleFoo() {
        fmt.Println("Output of this function.")
}

It also modifies godoc to read example code from _test.go files,
and include them in the HTML output with JavaScript-driven toggles.

It also implements testing of example functions with gotest.
The stdout/stderr is compared against the output comment on the
function.

This CL includes examples for the sort.Ints function and the
sort.SortInts type. After patching this CL in and re-building go/doc
and godoc, try
        godoc -http=localhost:6060
and visit http://localhost:6060/pkg/sort/

R=gri, r, rsc
CC=golang-dev
https://golang.org/cl/5137041