refactor(gtest): improve API and docs

[techraed]

• Rename ActorMailbox to UserMailbox, because there's no mailbox for programs
• Rework logs concept:
  Logs (Log and CoreLog, DecodedCoreLog) were introduced in the beginning of the gtest. Being useful, however, the naming was wrong. Also several types and API being 1) dependent on Into<Log>, 2) using Log to compare it to CoreLog, which itself is derived from user stored message, 3) having vague builder impl for Log, 4) having no utility for users of CoreLog made the API ambiguous. Now the concept is straightforward and clean. You have just UserMessageEvent derived from StoredMessages stored as events (not in mailbox) and EventBuilder to build those events and perform ops with them in BlockRunResult or UserMailbox
• Actualize crate docs, making it deeper, clarifying the concepts and usage

[semanticdiff-com]

Review changes with  

Changed Files

File	Status
gtest/src/manager/reservations.rs	96% smaller
gtest/src/state/mod.rs	89% smaller
gtest/src/manager/journal.rs	83% smaller
gtest/src/program.rs	61% smaller
gtest/src/system.rs	18% smaller
gtest/src/state/programs.rs	17% smaller
gtest/src/manager/block_exec.rs	17% smaller
gbuiltins/bls381/src/lib.rs	15% smaller
gtest/src/state/mailbox.rs	7% smaller
gtest/src/manager/memory.rs	1% smaller
Cargo.lock	Unsupported file format
gbuiltins/eth-bridge/src/lib.rs	0% smaller
gtest/Cargo.toml	Unsupported file format
gtest/src/artifacts.rs	0% smaller
gtest/src/builtins/bls12_381.rs	0% smaller
gtest/src/builtins/eth_bridge.rs	0% smaller
gtest/src/builtins/mod.rs	0% smaller
gtest/src/lib.rs	Unsupported file format
gtest/src/log.rs	0% smaller
gtest/src/manager.rs	0% smaller
gtest/src/manager/send_dispatch.rs	0% smaller
gtest/src/manager/task.rs	0% smaller
gtest/src/state/bridge.rs	0% smaller
gtest/src/state/mailbox/interface.rs	Unsupported file format