Replace slicec-cs by code generators written in C#

[bernardnormier]

Currently, slicec-cs is written in Rust, which is impractical for C# developers updating / improving code in icerpc-csharp: you need to be familiar with Rust to update the code generator, even though the focus of icerpc-csharp is naturally C#.

Protobuf provides an elegant solution to this issue: the protoc compiler launches plugins (in particular code-generator plugins) that can be written in any language.

We want to do the same for Slice:

• slicec (the compiler executable) remains in Rust but doesn't generate any code. Its focus is parsing.
• slicec communicates with plugins via stdint/stdout; in particular it serializes the AST using Slice2
• the slicec code generators for C# (base code generator and IceRPC code generator) are (re)written in C#