Incompatibility (TypeLoadException for `Typespec 0x1b000001`) with some Mono version built-in older Unity

[mickaelistria]

This comes after an analysis of LavaGang/MelonLoader#1202 .
In some Unity versions such as at least Unity-2017.4.8f1 to Unity-2018.4.24f1 (I didn't try to find the exact range of affected versions), trying to use Tomlet (and its consumers causes a crash:

Unhandled Exception: System.TypeInitializationException: An exception was thrown by the type initializer for Tomlet.TomlSerializationMethods ---> System.TypeLoadException: Could not load type 'Typespec 0x1b000001'.
  at (wrapper managed-to-native) System.MonoType:GetMethodsByName (string,System.Reflection.BindingFlags,bool,System.Type)
  at System.MonoType.GetMethodImpl (System.String name, BindingFlags bindingAttr, System.Reflection.Binder binder, CallingConventions callConvention, System.Type[] types, System.Reflection.ParameterModifier[] modifiers) [0x00000] in <filename unknown>:0 
  at System.Type.GetMethod (System.String name, BindingFlags bindingAttr) [0x00000] in <filename unknown>:0 
  at Tomlet.TomlSerializationMethods..cctor () [0x00000] in <filename unknown>:0 

The simplest reproducer I've seen so far is:

Test.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net35</TargetFramework>
    <GenerateRuntimeConfigurationFiles>true</GenerateRuntimeConfigurationFiles>
  </PropertyGroup>

  <ItemGroup>
    <!-- Same as in Overcooked: `monodis -assembly "$(pwd)/MelonLoader/net35/Tomlet.dll" | grep Version` -->
    <PackageReference Include="Samboy063.Tomlet" Version="6.2.0" />
  </ItemGroup>

</Project>

and Test.cs

using Tomlet;
using Tomlet.Attributes;

namespace Test {
	
	class MyClass {
	    [TomlProperty("name")]
	    public string Username { get; set; }
	}
	
	class Test {
	  static void Main(string[] args)
	  {
			var myClass = TomletMain.To<MyClass>("name=\"hello\""); //Requires a public, zero-argument constructor on MyClass.
			System.Console.WriteLine(myClass.Username);
		}
	}
}

then dotnet build and you get a nice /path/to/bin/Debug/net35/Test.exe. You can run it with a recent mono a
nd get the expected "hello" output.

Then, let's try to run it with the mono version built-in Unity: for the Unity Mono directory (eg ~/Downloads/Unity-2017.4.8f1/Editor/Data/Mono/bin-linux64, let's invoke MONO_PATH="$(pwd)/../lib/mono/2.0/" ./mono ~/sandbox/bin/Debug/net35/Test.exe
EXPECTED: hello
GOT:

type 0x1f not handled in do_mono_metadata_parse_type on image /home/mistria/sandbox/bin/Debug/net35/Tomlet.dll
The class Typespec 0x1b000001 could not be loaded, used in 

Unhandled Exception: System.TypeInitializationException: An exception was thrown by the type initializer for Tomlet.TomlSerializationMethods ---> System.TypeLoadException: Could not load type 'Typespec 0x1b000001'.
  at (wrapper managed-to-native) System.MonoType:GetMethodsByName (string,System.Reflection.BindingFlags,bool,System.Type)
  at System.MonoType.GetMethodImpl (System.String name, BindingFlags bindingAttr, System.Reflection.Binder binder, CallingConventions callConvention, System.Type[] types, System.Reflection.ParameterModifier[] modifiers) [0x00000] in <filename unknown>:0 
  at System.Type.GetMethod (System.String name, BindingFlags bindingAttr) [0x00000] in <filename unknown>:0 
  at Tomlet.TomlSerializationMethods..cctor () [0x00000] in <filename unknown>:0 
  --- End of inner exception stack trace ---
  at Tomlet.TomletMain.To (System.Type what, Tomlet.Models.TomlValue value, Tomlet.TomlSerializerOptions options) [0x00000] in <filename unknown>:0 
  at Tomlet.TomletMain.To (System.Type what, System.String tomlString, Tomlet.TomlSerializerOptions options) [0x00000] in <filename unknown>:0 
  at Tomlet.TomletMain.To[MyClass] (System.String tomlString, Tomlet.TomlSerializerOptions options) [0x00000] in <filename unknown>:0 
  at Test.Test.Main (System.String[] args) [0x00000] in <filename unknown>:0