Threadpool: remove stdlib dependencies

[mratsim]

This PR:

• removes dependency on Nim std/cpuinfo. Fix Remove Nim stdlib dependencies #291
  • implement available threads on OS, taking into account VMs with restricted cores.
  • export the procedure to C and Rust.
    • removes requiring Rust CSPRNG or NumCPUs to reduce supply chain attack surface, see Remove Nim stdlib dependencies #291.
    • We don't do the same in Go at the moment as Go has a runtime which provides the number of CPUs and crypt/rand has cryptographers reviewing and auditing the code. The cryptographic code is also part of OSS-Fuzz.
• Rework the Rust crate structure to introduce a constantine-core crate with common functionality like csprngs and threadpool.
• Outline topology detection goals:
  • get_num_threads_logical: with no OS/C stdlib dependency on x86 (all other arch need privileges to access specific registers)
  • get_num_threads_physical: with no OS/C stdlib dependency on x86 (all other arch need privileges to access specific registers)
  • get_num_threads_os: takes into account disabled cores (Virtual machines)
  • get_num_threads_process: Takes into account cgroup and schedaffinity

---

Previous targets

This addresses #291

• Counting cores
  • Allow counting physical cores on major platforms (supports FreeBSD, Linux, MacOS including iOS, Windows)
    • Allow counting physical cores without syscalls/IO on Linux x86 (no /proc or /sys read) for hardened trusted enclaves like SGX.
  • Allow counting available cores on Linux when running constrained by cgroups or CPUSET. Evaluate for docker/kubernetes
  • For completeness, allow counting logical cores without std/cpuinfo dependency
    • Allow counting logical cores without syscalls/IO on Linux x86 (no /proc or /sys read) for hardened trusted enclaves like SGX.
• Runtime trusted setup parsing without allocating a ref object or exceptions
  • Remove std/streams and just use C stdlib File stream API in

    constantine/constantine/trusted_setups/ethereum_kzg_srs.nim

    Lines 111 to 129 in 5f7ba18

    	proc loadTrustedSetup*(ctx: ptr EthereumKZGContext, filePath: string): TrustedSetupStatus =
    	static: doAssert cpuEndian == littleEndian, "Trusted setup creation is only supported on little-endian CPUs at the moment."
    	let f = try: openFileStream(filePath, fmRead)
    	except IOError, OSError: return tsMissingFile
    	defer:
    	try:
    	f.close()
    	except Exception: # For some reason close can raise a bare Exception.
    	quit "Unrecoverable error while closing trusted setup file."
    	try:
    	var buf = newSeqOfCap[byte](32)
    	var len = 0
    	buf.setLen(12)
    	len = f.readData(buf[0].addr, 12)

    done in Productionize KZG EIP-4844 #304