Errors following the guide with latest Nim version

[heysokam]

This part of the guide doesn't compile with latest version:
devel, haven't tried stable

# Line
var b = cast[ptr TGenericSeq](a)

# Output
Error: expression cannot be cast to TGenericSeq

Not error, but non-matching behavior that makes the guide hard to follow

# Code
var num :int
var p = num.addr
p[] = 30
echo num.addr.repr
echo num

# Output
ptr 30
30

To "fix" this, I wrote:

template reprByte(n :int) :string=
  ## Converts the address representation to its hex ByteAddress instead
  cast[ByteAddress](n.addr).repr

template reprOld(n :int) :string=
  ## Converts the address representation to a format that looks more similar to the old output
  n.addr.repr & " " & n.reprByte

I don't think these two templates fit a simple tutorial.
Just mentioning what I used to get a behavior similar to what the guide shows, so I could reason about the guide content.