cur vs len

• len holds max possible size before reallocation
• cur holds current length
• Speed!

    perl -MDevel::Peek -le '$a="simon"; $a = "nat"; Dump($a)'
    SV = PV(0x80f743c) at 0x810b4b4
      REFCNT = 1
      FLAGS = (POK,pPOK)
      PV = 0x80f6090 "nat"\0
      CUR = 3
      LEN = 6
  

• SvGROW(sv, len) extends sv to length, reallocating if necessary