Rename ZeroC.Slice to ZeroC.Slice.Codec

[bernardnormier]

I propose to rename the ZeroC.Slice namespace and NuGet package ZeroC.Slice.Codec.

This name would be consistent with the slice-codec name we use in Rust.

ZeroC.Slice.Codec (currently simply ZeroC.Slice) is about encoding/decoding Slice types. While this is an important component of Slice, it's not the only component, and we'll soon add:

• definitions produced by slicec (in ZeroC.Slice.Compiler)
• a nicer representation of these definitions (in ZeroC.Slice.Symbols)
• code-generator plug-ins (in ZeroC.Slice.CodeGen and IceRpc.Slice.CodeGen ?)

and more.