Add callgraphutil.WriteDOT

[picatz]

This PR adds the callgraphutil.WriteDOT function which can write a given *callgraph.Graph in DOT format to an io.Writer.

Example

Loading ./... packages from this repository, and using Graphviz's "Force-Directed Placement" layout (fdp) produces:

taint/callgraphutil/dot_test.go

Lines 168 to 193 in b107a34

	repo, _, err := cloneGitHubRepository(context.Background(), "picatz", "taint")
	if err != nil {
	t.Fatal(err)
	}
	pkgs, err := loadPackages(context.Background(), repo, "./...")
	if err != nil {
	t.Fatal(err)
	}
	mainFn, srcFns, err := loadSSA(context.Background(), pkgs)
	if err != nil {
	t.Fatal(err)
	}
	cg, err := loadCallGraph(context.Background(), mainFn, srcFns)
	if err != nil {
	t.Fatal(err)
	}
	output := &bytes.Buffer{}
	err = callgraphutil.WriteDOT(output, cg)
	if err != nil {
	t.Fatal(err)
	}

$ sfdp -Tsvg taint.dot -o taint.svg