fix allocator corruption for large sizes (#7338)

* fix allocator corruption for large sizes
* allow large chunks to coalesce and added test case
* use correct constants in MaxBigChunkSize

tests/system/tparams.nim

@@ -0,0 +1,18 @@
+import os
+import osproc
+import parseopt2
+import sequtils
+
+let argv = commandLineParams()
+
+if argv == @[]:
+  # this won't work with spaces
+  assert execShellCmd(getAppFilename() & " \"foo bar\" --aa:bar=a --a=c:d --ab -c --a[baz]:doo") == 0
+else:
+  let f = toSeq(getopt())
+  echo f.repr
+  assert f[0].kind == cmdArgument and f[0].key == "foo bar" and f[0].val == ""
+  assert f[1].kind == cmdLongOption and f[1].key == "aa" and f[1].val == "bar=a"
+  assert f[2].kind == cmdLongOption and f[2].key == "a=c" and f[2].val == "d"
+  assert f[3].kind == cmdLongOption and f[3].key == "ab" and f[3].val == ""
+  assert f[4].kind == cmdShortOption and f[4].key == "c" and f[4].val == ""