More human readable $(float)

The output matches that of Python (eg 1e100, not 1.0e100), but also
reflects locale (assuming it was set using setlocale() before).

lib/system/ansi_c.nim

@@ -106,10 +106,13 @@ proc c_fopen(filename, mode: cstring): C_TextFileStar {.
   importc: "fopen", header: "<stdio.h>".}
 proc c_fclose(f: C_TextFileStar) {.importc: "fclose", header: "<stdio.h>".}
 
-proc c_sprintf(buf, frmt: cstring) {.header: "<stdio.h>", 
+proc c_sprintf(buf, frmt: cstring): int {.header: "<stdio.h>", 
   importc: "sprintf", varargs, noSideEffect.}
   # we use it only in a way that cannot lead to security issues
 
+proc c_localeconv():ptr cstring {.header: "<locale.h>",
+  importc: "localeconv", noSideEffect.}
+
 proc c_fread(buf: pointer, size, n: int, f: C_BinaryFileStar): int {.
   importc: "fread", header: "<stdio.h>".}
 proc c_fseek(f: C_BinaryFileStar, offset: clong, whence: int): int {.

lib/system/repr.nim

@@ -17,12 +17,12 @@ proc reprFloat(x: float): string {.compilerproc.} = return $x
 
 proc reprPointer(x: pointer): string {.compilerproc.} =
   var buf: array [0..59, char]
-  c_sprintf(buf, "%p", x)
+  discard c_sprintf(buf, "%p", x)
   return $buf
 
 proc `$`(x: uint64): string =
   var buf: array [0..59, char]
-  c_sprintf(buf, "%llu", x)
+  discard c_sprintf(buf, "%llu", x)
   return $buf
 
 proc reprStrAux(result: var string, s: string) =

lib/system/sysstr.nim

@@ -250,10 +250,16 @@ proc nimIntToStr(x: int): string {.compilerRtl.} =
   for j in 0..i div 2 - 1:
     swap(result[j], result[i-j-1])
 
-proc nimFloatToStr(x: float): string {.compilerproc.} =
-  var buf: array [0..59, char]
-  c_sprintf(buf, "%#.16e", x)
-  return $buf
+proc nimFloatToStr(f: float): string {.compilerproc.} =
+  var buf: array [0..64, char]
+  var n:int = c_sprintf(buf, "%.16g", f)
+  for i in 0..n-1:
+    if buf[i] notin {'0'..'9','-'}:
+      return $buf
+  buf[n] = c_localeconv()[0]
+  buf[n+1] = '0'
+  buf[n+2] = '\0'
+  result = $buf
 
 proc nimInt64ToStr(x: int64): string {.compilerRtl.} =
   result = newString(sizeof(x)*4)