Do not over-commit fiber stacks on Windows

[HertzDevil]

Whenever a new fiber is spawned on Windows, currently Crystal allocates a fully committed 8 MB range of virtual memory. This commit charge stays until the stack becomes unused and reaped, even when most of the stack goes unused. Thus it is "only" possible to spawn several thousand fibers concurrently before the system runs out of virtual memory, depending on the total size of RAM and page files:

(0..).each do |i|
  print i, ' '
  spawn { sleep }
  sleep 1.millisecond
end

... 3362 3363 3364 Unhandled exception: VirtualAlloc: The paging file is too small for this operation to complete. (RuntimeError)
  from C:\crystal\src\crystal\system\win32\fiber.cr:24 in 'allocate_stack'
  from usr\test.cr:33 in 'allocate_stack'
  from C:\crystal\src\fiber\stack_pool.cr:45 in 'checkout'
  from C:\crystal\src\fiber.cr:89 in 'initialize'
  from C:\crystal\src\fiber.cr:86 in 'new'
  from C:\crystal\src\concurrent.cr:62 in 'spawn'
  from usr\test.cr:75 in '__crystal_main'
  from C:\crystal\src\crystal\main.cr:118 in 'main_user_code'
  from C:\crystal\src\crystal\main.cr:104 in 'main'
  from C:\crystal\src\crystal\main.cr:130 in 'main'
  from C:\crystal\src\crystal\system\win32\wmain.cr:37 in 'wmain'
  from D:\a\_work\1\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl:288 in '__scrt_common_main_seh'
  from C:\WINDOWS\System32\KERNEL32.DLL +75133 in 'BaseThreadInitThunk'
  from C:\WINDOWS\SYSTEM32\ntdll.dll +372520 in 'RtlUserThreadStart'

With this PR, for every fresh fiber stack, only the guard pages plus one extra initial page are committed. Spawning 100,000 idle fibers now consumes just around 7.4 GB of virtual memory, instead of 800 GB. Committed pages are also reset after a stack is returned to a pool and before it is retrieved again; this should be reasonably first, as decommitting pages doesn't alter the page contents.

Note that the guard pages reside immediately above the normal committed pages, not at the top of the whole reserved range. This is required for proper stack overflow detection.