fix: allow actually specifying fallibility

Siggingway/Fallibility.cs

@@ -0,0 +1,22 @@
+namespace Siggingway;
+
+/// <summary>
+/// The fallibility of a signature.
+/// </summary>
+public enum Fallibility {
+    /// <summary>
+    /// The fallibility of the signature is determined by the field/property's
+    /// nullability.
+    /// </summary>
+    Auto,
+
+    /// <summary>
+    /// The signature is fallible.
+    /// </summary>
+    Fallible,
+
+    /// <summary>
+    /// The signature is infallible.
+    /// </summary>
+    Infallible,
+}

Siggingway/Siggingway.cs

@@ -35,7 +35,14 @@ public static class Siggingway {
                 wasWrapped = true;
             }
 
-            var fallible = sig!.Fallible ?? info.IsNullable || wasWrapped;
+            var fallibility = sig!.Fallibility;
+            if (fallibility == Fallibility.Auto) {
+                fallibility = info.IsNullable || wasWrapped
+                    ? Fallibility.Fallible
+                    : Fallibility.Infallible;
+            }
+
+            var fallible = fallibility == Fallibility.Fallible;
 
             void Invalid(string message, bool prepend = true) {
                 var errorMsg = prepend

Siggingway/SignatureAttribute.cs

@@ -53,7 +53,7 @@ public sealed class SignatureAttribute : Attribute {
     /// exceptions instead. If fallibility is not specified, it is inferred
     /// based on if the field/property is nullable.
     /// </summary>
-    public bool? Fallible;
+    public Fallibility Fallibility = Fallibility.Auto;
 
     /// <summary>
     /// Create a <see cref="SignatureAttribute"/> with the given signature.