Add std:🧵:available_concurrency
This PR adds a counterpart to [C++'s `std:🧵:hardware_concurrency`](https://en.cppreference.com/w/cpp/thread/thread/hardware_concurrency) to Rust, tracking issue https://github.com/rust-lang/rust/issues/74479.
cc/ `@rust-lang/libs`
## Motivation
Being able to know how many hardware threads a platform supports is a core part of building multi-threaded code. In C++ 11 this has become available through the [`std:🧵:hardware_concurrency`](https://en.cppreference.com/w/cpp/thread/thread/hardware_concurrency) API. Currently in Rust most of the ecosystem depends on the [`num_cpus` crate](https://docs.rs/num_cpus/1.13.0/num_cpus/) ([no.35 in top 500 crates](https://docs.google.com/spreadsheets/d/1wwahRMHG3buvnfHjmPQFU4Kyfq15oTwbfsuZpwHUKc4/edit#gid=1253069234)) to provide this functionality. This PR proposes an API to provide access to the number of hardware threads available on a given platform.
__edit (2020-07-24):__ The purpose of this PR is to provide a hint for how many threads to spawn to saturate the processor. There's value in introducing APIs for NUMA and Windows processor groups, but those are intentionally out of scope for this PR. See: https://github.com/rust-lang/rust/pull/74480#issuecomment-662116186.
## Naming
Discussing the naming of the API on Zulip surfaced two options:
- `std:🧵:hardware_concurrency`
- `std:🧵:hardware_threads`
Both options seemed acceptable, but overall people seem to gravitate the most towards `hardware_threads`. Additionally `@jonas-schievink` pointed out that the "hardware threads" terminology is well-established and is used in among other the [RISC-V specification](https://riscv.org/specifications/isa-spec-pdf/) (page 20):
> A component is termed a core if it contains an independent instruction fetch unit. A RISC-V-compatible core might support multiple RISC-V-compatible __hardware threads__, or harts, through multithreading.
It's also worth noting that [the original paper introducing C++'s `std::thread` submodule](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2320.html) unfortunately doesn't feature any discussion on the naming of `hardware_concurrency`, so we can't use that to help inform our decision here.
## Return type
An important consideration `@joshtriplett` brought up is that we don't want to default to `1` for platforms where the number of available threads cannot be retrieved. Instead we want to inform the users of the fact that we don't know and allow them to handle that case. Which is why this PR uses `Option<NonZeroUsize>` as its return type, where `None` is returned on platforms where we don't know the number of hardware threads available.
The reasoning for `NonZeroUsize` vs `usize` is that if the number of threads for a platform are known, they'll always be at least 1. As evidenced by the example the `NonZero*` family of APIs may currently not be the most ergonomic to use, but improving the ergonomics of them is something that I think we can address separately.
## Implementation
`@Mark-Simulacrum` pointed out that most of the code we wanted to expose here was already available under `libtest`. So this PR mostly moves the internal code of libtest into a public API.
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