RFC: Base logging #24490

This is a straight copy - not useful by itself, but as a reference point for porting further changes.

* Remove 0.6 compatibility junk * Ensure logging macros can be used earlier in Base * Remove unnecessary types from core logging method defaults * max_log -> maxlog for log limiting * Beef up SimpleLogger a little to avoid temporary buffer async issues, respect maxlog, and make log colorization more consistent. It can be re-simplified again once there's a viable alternative in stdlib Logging. * Hash entire expression for log event ids. This makes ids more likely to be unique before deduplication, which makes them more likely to be stable.

This seems to be the simplest way to tackle two problems: * Tasks should inherit the logger state from the parent task. If not, all new tasks will use the global logger, which is unlikely to be what you want. In particular, the use of `@ async` in `Pkg` means we need something along these lines. * The cost of looking up the current log level in the Task local state dictionary is expensive when we want to discard a lot of potential Debug messages as quickly as possible.

For messages this was a somewhat mechanical upgrade assisted by Deprecations.jl. Some messages are lightly edited for consistency: * Where exceptions were interpolated, they're instead added as keyword arguments: `exception=ex` * An attempt has been made to punctuate log messages consistently with the convention from pkg: Short messages start with a capital, but have no period. * In places where the log message is generated using logic, the logic is pulled inside the message generation block. (Good practise from a scoping and efficiency point of view.) Notes on other changes: * `warn_once()` use has been replaced with the annotation `maxlog=1`. This depends on the logging backend to honor it, but makes things much more flexible. * `warnbanner()` use has been replaced with markdown formatted versions of `@ warn` * Avoid use display_error outside of REPL, thereby removing the log_error_to dependency * A few uses of bare println() have been replaced with logging * depwarns go to the new system, and use the frame address as the key for a speedy first pass at deduplication. A full depwarn report can be created in the future by collecting the log records * Port all logging tests to use test_logs where possible rather than reading streams. * Exceptions caught while executing commands on the REPL always go to the REPL output stream rather than being logged to the current logger, under the premise that this feedback is "normal output" for the REPL. Areas for further improvement: * A few remaining tests still use test_warn, as they call separate julia processes where the log records aren't yet forwarded transparently. * Tests for deprecated functionality are disabled in depwarn=error mode. It would be better to remove depwarn=error and rely on a logger instead.

To capture messages from Base, users should install a logger instead, for example, using `with_logger()`.

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RFC: Base logging #24490

RFC: Base logging #24490

Commits on Dec 15, 2017