Implement `seal_debug_message` (#792)

* Implement `seal_debug_message`

* Update docs

* Fmt

* Fix debug_print macro

* review: use newline char

Co-authored-by: Robin Freyler <robin.freyler@gmail.com>

* Fix example

* Revert to newline string

* Fmt

* Single call to debug_print for debug_println!

* Add missing ReturnCode, still need to handle it

* Inline debug_println!

* If logging is disabled then subsequent calls will be a no-op

* Fmt

* Fix missing error match in experimental off-chain

* Add safety comment to debug_message

* Only re-export ink_prelude::format, and explain

* Satisfy clippy

* Encapsulate DEBUG_ENABLED global in module

* Move seal_denug_message to unstable module

* Update unstable and safety comments

* Add more comments about the required features to be enabled on the node runtime

* Add `ink-debug` feature, make debug messages a noop if not enabled

* Fmt

* Noop macro formatting

* Enable debug printing for std

* Comment formatting

* Encapsulate static variable inside the function

* Fmt

* Remove debug_assert!(true) for disabled macros

Co-authored-by: Robin Freyler <robin.freyler@gmail.com>
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