use dragonbox algorithm; alternative to nim-lang#18008 (nim-lang#18139)

* use dragonbox algorithm; alternative to nim-lang#18008
* removed unsafe code

Author: Araq

changelog.md

@@ -76,6 +76,10 @@
 - `json` and `jsonutils` now serialize NaN, Inf, -Inf as strings, so that
   `%[NaN, -Inf]` is the string `["nan","-inf"]` instead of `[nan,-inf]` which was invalid json.
 
+- `system.addFloat` now uses the "Dragonbox" algorithm, which ensures correct roundtrips of floating point
+  numbers, that the minimum length representation of a floating point number is used and correct rounding.
+  Use `-d:nimLegacyAddFloat` for a transition period.
+
 - `strformat` is now part of `include std/prelude`.
 
 - Deprecated `proc reversed*[T](a: openArray[T], first: Natural, last: int): seq[T]` in `std/algorithm`.

compiler/condsyms.nim

@@ -134,3 +134,5 @@ proc initDefines*(symbols: StringTableRef) =
   defineSymbol("nimHasUnifiedTuple")
   defineSymbol("nimHasIterable")
   defineSymbol("nimHasTypeofVoid")
+
+  defineSymbol("nimHasDragonBox")