feat: Support singleton providers (#501)

* feat: Support singleton providers

This change adds support for provider functions that are
not reinvoked even if requested by multiple other providers.
Instead, their value is cached and reused between invocations.

To make this possible, we change how bindings are stored:
instead of just a function reference, we now store a binding object
which records whether the binding is a singleton,
and records the resolved singleton value (if any).

Resolves #500

* refac(bindings): hide singleton status

Don't require callAnyFunction to be aware of
whether a binding is a singleton or not.

context.go

@@ -120,10 +120,19 @@ func (c *Context) BindTo(impl, iface any) {
 // This is useful when the Run() function of different commands require different values that may
 // not all be initialisable from the main() function.
 //
-// "provider" must be a function with the signature func(...) (T, error) or func(...) T, where
-// ... will be recursively injected with bound values.
+// "provider" must be a function with the signature func(...) (T, error) or func(...) T,
+// where ... will be recursively injected with bound values.
 func (c *Context) BindToProvider(provider any) error {
-	return c.bindings.addProvider(provider)
+	return c.bindings.addProvider(provider, false /* singleton */)
+}
+
+// BindSingletonProvider allows binding of provider functions.
+// The provider will be called once and the result cached.
+//
+// "provider" must be a function with the signature func(...) (T, error) or func(...) T,
+// where ... will be recursively injected with bound values.
+func (c *Context) BindSingletonProvider(provider any) error {
+	return c.bindings.addProvider(provider, true /* singleton */)
 }
 
 // Value returns the value for a particular path element.
@@ -792,7 +801,7 @@ func (c *Context) RunNode(node *Node, binds ...any) (err error) {
 					methodt := t.Method(i)
 					if strings.HasPrefix(methodt.Name, "Provide") {
 						method := p.Method(i)
-						if err := methodBinds.addProvider(method.Interface()); err != nil {
+						if err := methodBinds.addProvider(method.Interface(), false /* singleton */); err != nil {
 							return fmt.Errorf("%s.%s: %w", t.Name(), methodt.Name, err)
 						}
 					}