Add external (debug) printing functionality about ink

[xlc]

Contract is able to raise custom events now, is it possible to repurpose it to emit logging events?
So there will be a a log macro that does nothing in release build but emit a logging event in debug build

[Robbepop]

For debuggability we cannot rely on events since they do not guarantee ordered dispatch.
So @ascjones is currently implementing the ext_println mechanics that are going to be dev chain only.

[pepyakin]

I think we could more or less rely on the ordering, however, the more apparent problem is that events are discarded when the execution fails which might not be useful for debugging failures : )

[Robbepop]

Commit 8517e0b implements this on the pDSL side. Can be closed now since it is also already merged on the Substrate side: https://github.com/paritytech/substrate/pull/2239/files