faster Run for known types

This is an alternative to PRs #160 and #165.
It's essentially the same as PR #165 except that it uses
generics to reduce the amount of duplicated code.

Instead of just amortizing the checking of the type, when
the argument type of the function passed to `Run` is known,
it bypasses the reflect-based code altogether.
We don't bother implementing the optimization on pre-generics
Go versions because those are end-of-lifetime anyway.

I've added an implementation-independent benchmark.

```
goos: linux
goarch: amd64
pkg: github.com/frankban/quicktest
cpu: Intel(R) Core(TM) i7-8565U CPU @ 1.80GHz
                           │      base      │               thisPR               │
                           │     sec/op     │   sec/op     vs base               │
CNewAndRunWithCustomType-8    1077.5n ±  5%   136.8n ± 6%  -87.30% (p=0.002 n=6)
CRunWithCustomType-8         1035.00n ± 11%   66.43n ± 3%  -93.58% (p=0.002 n=6)
geomean                        1.056µ         95.33n       -90.97%
```

bench_test.go

@@ -0,0 +1,29 @@
+package quicktest_test
+
+import (
+	"testing"
+
+	qt "github.com/frankban/quicktest"
+)
+
+func BenchmarkCNewAndRunWithCustomType(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		c := qt.New(customTForBenchmark{})
+		c.Run("test", func(c *qt.C) {})
+	}
+}
+
+func BenchmarkCRunWithCustomType(b *testing.B) {
+	c := qt.New(customTForBenchmark{})
+	for i := 0; i < b.N; i++ {
+		c.Run("test", func(c *qt.C) {})
+	}
+}
+
+type customTForBenchmark struct {
+	testing.TB
+}
+
+func (customTForBenchmark) Run(name string, f func(testing.TB)) bool {
+	return true
+}

quicktest.go

@@ -243,6 +243,10 @@ var (
 // A panic is raised when Run is called and the embedded concrete type does not
 // implement a Run method with a correct signature.
 func (c *C) Run(name string, f func(c *C)) bool {
+	r, ok := fastRun(c, name, f)
+	if ok {
+		return r
+	}
 	badType := func(m string) {
 		panic(fmt.Sprintf("cannot execute Run with underlying concrete type %T (%s)", c.TB, m))
 	}

run_go1.18.go

@@ -0,0 +1,41 @@
+// Licensed under the MIT license, see LICENSE file for details.
+
+//go:build go1.18
+// +build go1.18
+
+package quicktest
+
+import "testing"
+
+// fastRun implements c.Run for some known types.
+// It returns the result of calling c.Run and also reports
+// whether it was able to do so.
+func fastRun(c *C, name string, f func(c *C)) (bool, bool) {
+	switch t := c.TB.(type) {
+	case runner[*testing.T]:
+		return fastRun1(c, name, f, t), true
+	case runner[*testing.B]:
+		return fastRun1(c, name, f, t), true
+	case runner[*C]:
+		return fastRun1(c, name, f, t), true
+	case runner[testing.TB]:
+		// This case is here mostly for benchmarking, because
+		// it's hard to create a working concrete instance of *testing.T
+		// that isn't part of the outer tests.
+		return fastRun1(c, name, f, t), true
+	}
+	return false, false
+}
+
+type runner[T any] interface {
+	Run(name string, f func(T)) bool
+}
+
+func fastRun1[T testing.TB](c *C, name string, f func(*C), t runner[T]) bool {
+	return t.Run(name, func(t2 T) {
+		c2 := New(t2)
+		defer c2.Done()
+		c2.SetFormat(c.getFormat())
+		f(c2)
+	})
+}

run_go1.18_test.go

@@ -0,0 +1,60 @@
+// Licensed under the MIT license, see LICENSE file for details.
+
+//go:build go1.18
+// +build go1.18
+
+package quicktest_test
+
+import (
+	"reflect"
+	"testing"
+
+	qt "github.com/frankban/quicktest"
+)
+
+type customT2[T testing.TB] struct {
+	testing.TB
+}
+
+func (t *customT2[T]) Run(name string, f func(T)) bool {
+	f(*new(T))
+	return true
+}
+
+func (t *customT2[T]) rtype() reflect.Type {
+	return reflect.TypeOf((*T)(nil)).Elem()
+}
+
+type otherTB struct {
+	testing.TB
+}
+
+func TestCRunCustomTypeWithNonMatchingRunSignature(t *testing.T) {
+	// Note: this test runs only on >=go1.18 because there isn't any
+	// code that specializes on this types that's enabled on versions before that.
+	tests := []interface {
+		testing.TB
+		rtype() reflect.Type
+	}{
+		&customT2[*testing.T]{},
+		&customT2[*testing.B]{},
+		&customT2[*qt.C]{},
+		&customT2[testing.TB]{},
+		&customT2[otherTB]{},
+	}
+	for _, test := range tests {
+		t.Run(test.rtype().String(), func(t *testing.T) {
+			c := qt.New(test)
+			called := 0
+			c.Run("test", func(c *qt.C) {
+				called++
+				if test.rtype().Kind() != reflect.Interface && reflect.TypeOf(c.TB) != test.rtype() {
+					t.Errorf("TB isn't expected type (want %v got %T)", test.rtype(), c.TB)
+				}
+			})
+			if got, want := called, 1; got != want {
+				t.Errorf("subtest was called %d times, not once", called)
+			}
+		})
+	}
+}

run_legacy.go

@@ -0,0 +1,10 @@
+// Licensed under the MIT license, see LICENSE file for details.
+
+//go:build !go1.18
+// +build !go1.18
+
+package quicktest
+
+func fastRun(c *C, name string, f func(c *C)) (bool, bool) {
+	return false, false
+}