improvements to addInt and $ for integer types (#18592)

* improvements to $(SomeInteger) and addInt
* remove mIntToStr, mInt64ToStr
* improvements
* fix tests/pragmas/tinjectstmt.nim; the diff is harmless, cgen code is identical with -d:danger or debug mode
* rm tests/system/tstrmantle.nim
* revert compiler/jsgen.nim for -d:nimVersion140

lib/system/strmantle.nim

@@ -43,32 +43,6 @@ proc hashString(s: string): int {.compilerproc.} =
   h = h + h shl 15
   result = cast[int](h)
 
-proc addInt*(result: var string; x: int64) =
-  ## Converts integer to its string representation and appends it to `result`.
-  ##
-  ## .. code-block:: Nim
-  ##   var
-  ##     a = "123"
-  ##     b = 45
-  ##   a.addInt(b) # a <- "12345"
-  var num: uint64
-
-  if x < 0:
-    if x == low(int64):
-      num = uint64(x)
-    else:
-      num = uint64(-x)
-    let base = result.len
-    setLen(result, base + 1)
-    result[base] = '-'
-  else:
-    num = uint64(x)
-  addIntImpl(result, num)
-
-proc nimIntToStr(x: int): string {.compilerRtl.} =
-  result = newStringOfCap(sizeof(x)*4)
-  result.addInt x
-
 proc c_strtod(buf: cstring, endptr: ptr cstring): float64 {.
   importc: "strtod", header: "<stdlib.h>", noSideEffect.}
 
@@ -240,10 +214,6 @@ proc nimParseBiggestFloat(s: string, number: var BiggestFloat,
 when defined(nimHasInvariant):
   {.pop.} # staticBoundChecks
 
-proc nimInt64ToStr(x: int64): string {.compilerRtl.} =
-  result = newStringOfCap(sizeof(x)*4)
-  result.addInt x
-
 proc nimBoolToStr(x: bool): string {.compilerRtl.} =
   return if x: "true" else: "false"