Add something like rustc's -Z time-passes

[fitzgen]

This would give a high level overview of how long each of our phases are taking, and help inform us where we should generally be focusing our performance investigations on various test cases.

[fitzgen]

Specifically, I'm imagining

• a new module that provides RAII types for timing start and end of some phase, and printing the timed value in the Drop implementation,
• the ability to turn those RAII timers on/off dynamically via a command line flag and builder option,
• and then instrumentation of the code base with these RAII timers for all of:
  • libclang -> IR parsing (bindgen::parse)
  • each of our analyses (eg bindgen::ir::BindgenContext::compute_cannot_derive_debug)
  • bindgen::ir::BindgenContext::process_replacements
  • bindgen::ir::BindgenContext::compute_whitelisted_and_codegen_items
  • IR -> Rust source code (bindgen::codegen::codegen)

[jhod0]

@fitzgen I implemented something along these lines, let me know if it's what you had in mind (PR #938)