tar/asm: DiscardFilePutter and stub disassemble

Have a bit-bucket FilePutter, for when it does not matter.

Beginning thoughts on disassembly, but it has things that need thought.
Mostly comments in the function for now.

tar/asm/assemble_test.go

@@ -0,0 +1,9 @@
+package asm
+
+import "testing"
+
+func TestNewOutputTarStream(t *testing.T) {
+	// TODO disassembly
+	fgp := NewBufferFileGetPutter()
+	_ = NewOutputTarStream(fgp, nil)
+}