feat: add p/demo/pausable (gnolang#1328)

## Description


This PR adds the `Pausable` package to p/demo, allowing realms to have
pausability. It relies on the already existing `Ownable` package for
access control, gnolang#1314.

Inspired by OpenZeppelin's
[Pausable](https://github.com/OpenZeppelin/openzeppelin-contracts/blob/v4.9.3/contracts/security/Pausable.sol).

<details><summary>Contributors' checklist...</summary>

- [x] Added new tests, or not needed, or not feasible
- [x] Provided an example (e.g. screenshot) to aid review or the PR is
self-explanatory
- [x] Updated the official documentation or not needed
- [x] No breaking changes were made, or a `BREAKING CHANGE: xxx` message
was included in the description
- [x] Added references to related issues and PRs
- [ ] Provided any useful hints for running manual tests
- [ ] Added new benchmarks to [generated
graphs](https://gnoland.github.io/benchmarks), if any. More info
[here](https://github.com/gnolang/gno/blob/master/.benchmarks/README.md).
</details>

---------

Co-authored-by: Morgan Bazalgette <morgan@morganbaz.com>

examples/gno.land/p/demo/pausable/gno.mod

@@ -0,0 +1,3 @@
+module gno.land/p/demo/pausable
+
+require gno.land/p/demo/ownable v0.0.0-latest

examples/gno.land/p/demo/pausable/pausable.gno

@@ -0,0 +1,49 @@
+package pausable
+
+import "gno.land/p/demo/ownable"
+
+type Pausable struct {
+	*ownable.Ownable
+	paused bool
+}
+
+// New returns a new Pausable struct with non-paused state as default
+func New() *Pausable {
+	return &Pausable{
+		Ownable: ownable.New(),
+		paused:  false,
+	}
+}
+
+// NewFromOwnable is the same as New, but with a pre-existing top-level ownable
+func NewFromOwnable(ownable *ownable.Ownable) *Pausable {
+	return &Pausable{
+		Ownable: ownable,
+		paused:  false,
+	}
+}
+
+// IsPaused checks if Pausable is paused
+func (p Pausable) IsPaused() bool {
+	return p.paused
+}
+
+// Pause sets the state of Pausable to true, meaning all pausable functions are paused
+func (p *Pausable) Pause() error {
+	if err := p.CallerIsOwner(); err != nil {
+		return err
+	}
+
+	p.paused = true
+	return nil
+}
+
+// Unpause sets the state of Pausable to false, meaning all pausable functions are resumed
+func (p *Pausable) Unpause() error {
+	if err := p.CallerIsOwner(); err != nil {
+		return err
+	}
+
+	p.paused = false
+	return nil
+}

examples/gno.land/p/demo/pausable/pausable_test.gno

@@ -0,0 +1,77 @@
+package pausable
+
+import (
+	"std"
+	"testing"
+
+	"gno.land/p/demo/ownable"
+)
+
+var (
+	firstCaller  = std.Address("g1l9aypkr8xfvs82zeux486ddzec88ty69lue9de")
+	secondCaller = std.Address("g127jydsh6cms3lrtdenydxsckh23a8d6emqcvfa")
+)
+
+func TestNew(t *testing.T) {
+	std.TestSetOrigCaller(firstCaller)
+
+	result := New()
+
+	if result.paused != false {
+		t.Fatalf("Expected result to be unpaused, got %t\n", result.paused)
+	}
+
+	if result.Owner() != firstCaller {
+		t.Fatalf("Expected %s, got %s\n", firstCaller, result.Owner())
+	}
+}
+
+func TestNewFromOwnable(t *testing.T) {
+	std.TestSetOrigCaller(firstCaller)
+	o := ownable.New()
+
+	std.TestSetOrigCaller(secondCaller)
+	result := NewFromOwnable(o)
+
+	if result.Owner() != firstCaller {
+		t.Fatalf("Expected %s, got %s\n", firstCaller, result.Owner())
+	}
+}
+
+func TestSetUnpaused(t *testing.T) {
+	std.TestSetOrigCaller(firstCaller)
+
+	result := New()
+	result.Unpause()
+
+	if result.IsPaused() {
+		t.Fatalf("Expected result to be unpaused, got %t\n", result.IsPaused())
+	}
+}
+
+func TestSetPaused(t *testing.T) {
+	std.TestSetOrigCaller(firstCaller)
+
+	result := New()
+	result.Pause()
+
+	if !result.IsPaused() {
+		t.Fatalf("Expected result to be paused, got %t\n", result.IsPaused())
+	}
+}
+
+func TestIsPaused(t *testing.T) {
+	std.TestSetOrigCaller(firstCaller)
+
+	result := New()
+
+	if result.IsPaused() {
+		t.Fatalf("Expected result to be unpaused, got %t\n", result.IsPaused())
+	}
+
+	result.Pause()
+
+	if !result.IsPaused() {
+		t.Fatalf("Expected result to be paused, got %t\n", result.IsPaused())
+	}
+}