Taking the example of the AMF Set ID, the previous version of the macro
used another macro to encode the number, then marked the last 6 bits in
the bit string (of length 16/2 bytes) as unused. This is wrong, because,
assuming AMF Set ID=3, it basically truncates the number by encoding all
16 bits and cutting off the last 6, like this:

  ------------------- encode 16 bits
  0000 0000 0000 0011
  ------------        take these 10 bits

So what remains is these bit positions of the number:
(16,15,14,13,12,11,10,9,8,7) -- the last 6 bits are cut (and the first 6
are either 0 or garbage, because they don't exist).

Instead, manually encode bits (10,9,8,7,6,5,4,3) in the first byte, and
(2,1) in the second byte of the bit string, like so:

  ------------        encode 10 bits
  0000 0000 1100 0000
  ------------        take these 10 bits

Do the same with the pointer.

The AMF Region ID above is correct, as it is exactly 8 bits.