Store more information in headers

[Mark-Simulacrum]

Taking inspiration from perf on Linux via the description of it's file format, we thought that storing the number of threads, CPU type, and available memory on the system would be a good start. All of these should be optional, though.

Relevant discussion on Zulip: https://rust-lang.zulipchat.com/#narrow/stream/187831-t-compiler.2Fwg-self-profile/topic/process-id.20in.20events/near/165803887

[bjorn3]

Would also be nice to include the version string of the rustc that is being profiled.