[registrar] Increase code sharing in generated `native_to_managed_trampoline_*` functions (#7221)

Several functions were 100% identical except for their signature. However
the signature were over-specialized and simplifying them allows the
(already) present code merge feature to achieve much better results.

Final size impact will vary based on the API used (both the managed and
native linker can remove some) but the more API you use the more likely
the application included duplicate code.

**Before**

`tools/mtouch/Xamarin.iOS.registrar.ios.x86_64.m` has `native_to_managed_trampoline_[1..379]`

```
find tools/ -name Xamarin.*.a | xargs ls -l
-rw-r--r--  1 poupou  staff  4143148 11 Oct 15:09 tools//mmp/Xamarin.Mac.registrar.full.a
-rw-r--r--  1 poupou  staff  4139052 11 Oct 15:09 tools//mmp/Xamarin.Mac.registrar.full.x86_64.a
-rw-r--r--  1 poupou  staff  4143160 11 Oct 15:09 tools//mmp/Xamarin.Mac.registrar.mobile.a
-rw-r--r--  1 poupou  staff  4139064 11 Oct 15:09 tools//mmp/Xamarin.Mac.registrar.mobile.x86_64.a
-rw-r--r--  1 poupou  staff  4521752 11 Oct 15:09 tools//mtouch/Xamarin.iOS.registrar.ios.i386.a
-rw-r--r--  1 poupou  staff  9240416 11 Oct 15:09 tools//mtouch/Xamarin.iOS.registrar.ios.simulator.a
-rw-r--r--  1 poupou  staff  4714336 11 Oct 15:09 tools//mtouch/Xamarin.iOS.registrar.ios.x86_64.a
```

**After**

`tools/mtouch/Xamarin.iOS.registrar.ios.x86_64.m` has `native_to_managed_trampoline_[1..132]`

```
find tools/ -name Xamarin.*.a | xargs ls -l
-rw-r--r--  1 poupou  staff  3723012 11 Oct 14:57 tools//mmp/Xamarin.Mac.registrar.full.a
-rw-r--r--  1 poupou  staff  3718916 11 Oct 14:57 tools//mmp/Xamarin.Mac.registrar.full.x86_64.a
-rw-r--r--  1 poupou  staff  3723016 11 Oct 14:57 tools//mmp/Xamarin.Mac.registrar.mobile.a
-rw-r--r--  1 poupou  staff  3718920 11 Oct 14:57 tools//mmp/Xamarin.Mac.registrar.mobile.x86_64.a
-rw-r--r--  1 poupou  staff  3995520 11 Oct 14:57 tools//mtouch/Xamarin.iOS.registrar.ios.i386.a
-rw-r--r--  1 poupou  staff  8195748 11 Oct 14:57 tools//mtouch/Xamarin.iOS.registrar.ios.simulator.a
-rw-r--r--  1 poupou  staff  4193956 11 Oct 14:57 tools//mtouch/Xamarin.iOS.registrar.ios.x86_64.a
```

tools/common/StaticRegistrar.cs

@@ -2326,6 +2326,19 @@ namespace Registrar {
 			return suggestion;
 		}
 
+		// Some declarations can be generalized to NSObject for its subclasses
+		// (and System.String too as we convert it into an NSString)
+		// since the generated code, except the function signature, is identical anyway
+		string ToSimpleObjCParameterType (TypeReference type, string descriptiveMethodName, List<Exception> exceptions, MemberReference inMethod)
+		{
+			var byref = type.IsByReference;
+			var t = byref ? type.GetElementType () : type;
+			if (t.Inherits ("Foundation", "NSObject") || t.Is ("System", "String"))
+				return byref ? "id*" : "id";
+
+			return ToObjCParameterType (type, descriptiveMethodName, exceptions, inMethod);
+		}
+
 		string ToObjCParameterType (TypeReference type, string descriptiveMethodName, List<Exception> exceptions, MemberReference inMethod)
 		{
 			TypeDefinition td = ResolveType (type);
@@ -3215,7 +3228,7 @@ namespace Registrar {
 					rettype = "double";
 					break;
 				default:
-					rettype = ToObjCParameterType (method.NativeReturnType, descriptiveMethodName, exceptions, method.Method);
+					rettype = ToSimpleObjCParameterType (method.NativeReturnType, descriptiveMethodName, exceptions, method.Method);
 					break;
 				}
 				break;
@@ -3962,7 +3975,7 @@ namespace Registrar {
 			var objc_signature = new StringBuilder ().Append (rettype).Append (":");
 			if (method.Method.HasParameters) {
 				for (int i = 0; i < method.NativeParameters.Length; i++)
-					objc_signature.Append (ToObjCParameterType (method.NativeParameters [i], descriptiveMethodName, exceptions, method.Method)).Append (":");
+					objc_signature.Append (ToSimpleObjCParameterType (method.NativeParameters [i], descriptiveMethodName, exceptions, method.Method)).Append (":");
 			}
 
 			Body existing;
@@ -3985,7 +3998,7 @@ namespace Registrar {
 					methods.Append (rettype).Append (" ").Append (b.Name).Append (" (id self, SEL _cmd, MonoMethod **managed_method_ptr");
 					var pcount = method.Method.HasParameters ? method.NativeParameters.Length : 0;
 					for (int i = (isInstanceCategory ? 1 : 0); i < pcount; i++) {
-						methods.Append (", ").Append (ToObjCParameterType (method.NativeParameters [i], descriptiveMethodName, exceptions, method.Method));
+						methods.Append (", ").Append (ToSimpleObjCParameterType (method.NativeParameters [i], descriptiveMethodName, exceptions, method.Method));
 						methods.Append (" ").Append ("p").Append (i.ToString ());
 					}
 					if (isCtor)