Support for C# 9 function pointers

[dgrunwald]

This PR adds support for C# 9.0 function pointer types (delegate*<...>) to the type system.

• Function pointer types can be decoded from signatures
• The calli translation is changed to convert to a function pointer type if necessary (not always inserting a cast).
• The calli translation now supports managed references for parameter or return types.
• The ldftn translation is changed to use the C# 9 "address of method-group" syntax.
• Support for function pointer types added to type inference.
• Conversions between IntPtr and function pointers are supported.
• Variance conversions for function pointers are supported.

Not yet implemented here:
[ ] Taking the address of a static local function
[ ] The upcoming syntax change for calling conventions
[ ] Custom calling convention support