LoadIndirect support for char and bool

The C# compiler uses `Ldind_` for references and pointers to `char` and `bool`, it should be possible to do the same with Sigil.

```csharp
static bool BoolExample(ref bool a) { return a; }
// IL_0000:  ldarg.0     
// IL_0001:  ldind.i1    
// IL_0002:  ret         

static char CharExample(ref char a) { return a; }
// IL_0000:  ldarg.0     
// IL_0001:  ldind.u2    
// IL_0002:  ret         
```

Usage with this change (see unit tests for complete example):
```csharp
e1.LoadIndirect<bool>();
e2.LoadIndirect<char>();
```

Sigil is a great library, thanks for making this available!