Redesign the logging subsystem

[brson]

It is quite old and primitive at this stage, being mostly implemented in the runtime. With our new-found Rustic powers we should be able to come up with a more robust design.

Sub-bugs to handle along the way:

• RUST_LOG=::help should be sorted #8949 RUST_LOG=::help should be sorted
• Remove the longest-match logic in module-name matching for logging #6032 path prefix / full name RUST_LOG specs
• Trace task names in logging macros #2672 trace task names when logging
• multiple logging sinks #7805 multiple logging sinks
• Make logging work with new runtime code #5021 integration with new runtime code
• Monomorphised log statements have the wrong path in the crate_map #3046 monomorphized fns get wrong logging name
• Warn when RUST_LOG contains a name that isn't a valid path #2038 warn on nonmatching RUST_LOG
• Symbolic log levels, not just numbers #6031 symbolic log levels
• crate name in log map is taken from name of output binary, not the crate name #3346 wrong name for logging (uses binary, not crate name)
• logging functions handle unicode poorly #2154 unicode logging
• RUST_LOG=error should turn on error-level logging for all modules #6033 turning on everything with RUST_LOG=error
• rustc out of stack when RUST_LOG=rustc #5958 and RUST_LOG setting causes huge memory leak when building rustc #6558 OOM when running rustc with RUST_LOG on
• Custom polymorphic logging #2366 custom polymorphic logging

[nikomatsakis]

Note related request #4397

[catamorphism]

Note related request #3309

[emberian]

Hitting for triage

[emberian]

A log_push! and log_pop! would be pretty cool, like what q3 has (https://github.com/Jeaye/q3/blob/master/src/util/log_macros.rs). It lets you group a bunch of logs, claiming ownership of them.