Decode NUMBER into float32 and float64 fields

[magiconair]

The decoder does not allow to decode a NUMBER into a float32/float64 field.

func TestDecodeFloat(t *testing.T) {
	type X struct {
		A float64
	}

	var x X
	if err := hcl.Decode(&x, "a=2"); err != nil {
		t.Fatal(err)
	}
}

fails with

--- FAIL: TestDecodeFloat (0.00s)
	main_test.go:19: At 1:3: root.A: unknown type *ast.LiteralType