go/doc, godoc, gotest: support for reading example documentation

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

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
12 files changed