[marshal methods] Add support for wrapping the char type

[grendello]

System.Char is an unblittable type and, as such, it has to be treated
specially by the marshal methods wrapper generator to properly cast it
to/from a matching blittable type. In this case the latter is ushort,
since both on the Java and Managed side, char is a 16-bit entity.

[jonpryor]

Why is char not blittable? Documentation lists it as "sometimes" blittable (which itself is worrying, to be fair): https://learn.microsoft.com/en-us/dotnet/standard/native-interop/best-practices#blittable-types-when-runtime-marshalling-is-enabled

Then there's the following section Blittable types when runtime marshalling is disabled:

char is always passed as a 2-byte value

which implies to me that char is supported as a blittable type, and (when runtime marshaling is disabled) is marshaled as a 16-bit construct, which is perfect as that matches Java's char type.

[grendello]

@jonpryor I followed this https://learn.microsoft.com/en-us/dotnet/framework/interop/blittable-and-non-blittable-types

[jonpryor]

@grendello: based on the URL scheme and the "banner" at the top of the link, I believe that the docs you referred to are for .NET Framework, not .NET. As such, I do not believe that this special-casing of char is required.

[grendello]

@jonpryor it doesn't change for .NET https://learn.microsoft.com/en-us/dotnet/standard/native-interop/best-practices#blittable-types-when-runtime-marshalling-is-enabled, and also the docs in the previous link refer to the .NET Framework docs, so they're still valid I think.