Only copy strings to their size, not capacity

Capacity may be much bigger, so we end up with strings that are much
larger than they have to be and have to copy more as well.

lib/system/sysstr.nim

@@ -77,13 +77,13 @@ proc copyString(src: NimString): NimString {.compilerRtl.} =
     if (src.reserved and seqShallowFlag) != 0:
       result = src
     else:
-      result = rawNewString(src.space)
+      result = rawNewString(src.len)
       result.len = src.len
       c_memcpy(result.data, src.data, (src.len + 1) * sizeof(char))
 
 proc copyStringRC1(src: NimString): NimString {.compilerRtl.} =
   if src != nil:
-    var s = src.space
+    var s = src.len
     if s < 8: s = 7
     when declared(newObjRC1):
       result = cast[NimString](newObjRC1(addr(strDesc), sizeof(TGenericSeq) +