Make most of Message functions non-virtual to reduce generated code size

[stepancheg]

Currently, Message has 10 virtual functions which have default implementations which result in increased generated code size.

An example of such function is write_to.

These functions should be non-virtual, and invocable with <M : Message> and &Message.

Possible solutions:

• Wait until Method resolution for trait object types does not prefer inherent methods rust-lang/rust#51402 (comment) is resolved. With this feature implemented it will be possible to implement functions like:

trait Message {
    fn write_to(&self, ...) where Self: Sized { ... }
}

impl dyn Message {
    // copy-paste of above, that's fine
    fn write_to(&self, ...) { ... }
}

• Add write_to function to MessageExt crate and add MessageExt to crate prelude.

Custom preludes could help here rust-lang/rfcs#890

• Inherent traits would help Inherent trait implementations rust-lang/rfcs#1880

[stepancheg]

In master, Message functions are not virtual, but additional MessageDyn trait is introduced for dynamic invocation

[stepancheg]

This is done in version 3, and won't be ported to branch 2.