Logging: Replace tracing with an efficient logging implementation, with 0-runtime cost for disabled log levels. (https://www.pivotaltracker.com/story/show/183755412)
Profiling: Support submitting `profiler` events to the User Timing Web API, so that measurements can be viewed directly in the browser. (https://www.pivotaltracker.com/story/show/184003550)
# Important Notes
Logging interface:
- The macros (`warn!`, etc.) now take standard `format_args!` arguments (the tracing implementations accepted a broader syntax).
- Compile-time log levels can now be set through the CLI, like so:
`./run ide start --log-level=trace --uncollapsed-log-level=info`
Profiling:
- The hotkey Ctrl+Alt+Shift+P submits all `profiler` events logged since the application was loaded to the Web API, so that they can then be viewed with the browser's developer tools. Note that standard tools are not able to represent async task lifetimes or metadata; this is a convenient interface to a subset of `profiler` data.
- As an alternative interface, a runtime flag enables continuous measurement submission. In the browser it can be set through a URL parameter, like http://localhost:8080/?emit_user_timing_measurements=true. Note that this mode significantly impacts performance.
