RuntimeEnvironmentHelper: remove Lazy<> for basic platform checks

[fowl2]

IsOSPlatform() is fast, so remove all the overhead from using a bunch of Lazy<>.

IsLinux also returns true on BSD, not sure if that's documented, but switched to conditional compilation to remove an allocation.

I'm not sure if Mono is still supported - if it's not, ripping all that code out might be a good follow up.

[nkolev92]

Let's keep the BSD check on framework too.

[nkolev92]

Let's just add back the FreeBSD detection.

While mono is technically out of support, I don't want to break anything accidentally.

[nkolev92]

It's faster assuming this was called once, but we actually call these methods many many times, for example:

NuGet.Client/src/NuGet.Core/NuGet.Common/PathUtil/PathUtility.cs

Lines 158 to 170 in 1ee6830

	public static bool IsDirectorySeparatorChar(char ch)
	{
	if (RuntimeEnvironmentHelper.IsWindows)
	{
	// Windows has both '/' and '\' as valid directory separators.
	return (ch == Path.DirectorySeparatorChar ||
	ch == Path.AltDirectorySeparatorChar);
	}
	else
	{
	return ch == Path.DirectorySeparatorChar;
	}
	}

I ran the benchmarks:

● Benchmark results (10,000 iterations per call, on Hyper-V):

┌───────────────┬─────────────────┬───────────────────┬─────────────┐
│ Method │ Baseline (Lazy) │ Proposed (direct) │ Δ │
├───────────────┼─────────────────┼───────────────────┼─────────────┤
│ IsWindows │ 8.59 µs │ 6.74 µs │ -22% ✅ │
├───────────────┼─────────────────┼───────────────────┼─────────────┤
│ IsMacOSX │ 6.61 µs │ 8.14 µs │ +23% ❌ │
├───────────────┼─────────────────┼───────────────────┼─────────────┤
│ IsLinux │ 6.56 µs │ 3.93 µs │ -40% ✅ │
├───────────────┼─────────────────┼───────────────────┼─────────────┤
│ IsMono │ 11.53 µs │ 3.91 µs │ -66% ✅ │

[nkolev92]

I didn't really add a summary here :D

This is me to say the change is good, but I think it would've been great to see this type of benchmark in the PR body to begin with.