consistent floating point output (#12219)

* unify float printing
* makes tests green

lib/system/formatfloat.nim

@@ -0,0 +1,59 @@
+#
+#
+#            Nim's Runtime Library
+#        (c) Copyright 2019 Nim contributors
+#
+#    See the file "copying.txt", included in this
+#    distribution, for details about the copyright.
+#
+
+proc c_sprintf(buf, frmt: cstring): cint {.header: "<stdio.h>",
+                                    importc: "sprintf", varargs, noSideEffect.}
+
+proc writeToBuffer(buf: var array[65, char]; value: cstring) =
+  var i = 0
+  while value[i] != '\0':
+    buf[i] = value[i]
+    inc i
+
+proc writeFloatToBuffer*(buf: var array[65, char]; value: BiggestFloat): int =
+  ## This is the implementation to format floats in the Nim
+  ## programming language. The specific format for floating point
+  ## numbers is not specified in the Nim programming language and
+  ## might change slightly in the future, but at least wherever you
+  ## format a float, it should be consistent.
+  ##
+  ## returns the amount of bytes written to `buf` not counting the
+  ## terminating '\0' character.
+  ##
+  ## * `buf` - A buffer to write into. The buffer does not need to be
+  ##           initialized and it will be overridden.
+  ##
+  var n: int = c_sprintf(addr buf, "%.16g", value)
+  var hasDot = false
+  for i in 0..n-1:
+    if buf[i] == ',':
+      buf[i] = '.'
+      hasDot = true
+    elif buf[i] in {'a'..'z', 'A'..'Z', '.'}:
+      hasDot = true
+  if not hasDot:
+    buf[n] = '.'
+    buf[n+1] = '0'
+    buf[n+2] = '\0'
+    result = n + 2
+  else:
+    result = n
+  # On Windows nice numbers like '1.#INF', '-1.#INF' or '1.#NAN'
+  # of '-1.#IND' are produced.
+  # We want to get rid of these here:
+  if buf[n-1] in {'n', 'N', 'D', 'd'}:
+    writeToBuffer(buf, "nan")
+    result = 3
+  elif buf[n-1] == 'F':
+    if buf[0] == '-':
+      writeToBuffer(buf, "-inf")
+      result = 4
+    else:
+      writeToBuffer(buf, "inf")
+      result = 3