Specify the range of i and k in wallet paths.

There has been some disagreement in implementations as to whether:
  "m/0'/1" == "m/2147483648/1"

The use of strings to denote wallet paths is outside the scope
of BIP32; but it will become common.  Allowing for this equivalence
creates some subtle bugs:
  a) most implementations are using the presence of a ' in the path
     to indicate that it contains a prime derivation.  If we allow
     for >31bit numbers, this naive check becomes invalid.
  b) paths now have multiple, equivalent identifiers, making
     normalization necessary.
  c) anyone creating new keypairs needs to be aware of this when
     selecting i values, and it is just subtle.