Syntactic sugar for magic 0x5e1f constant

[robknight]

In Podlang, we want to be able to write SELF, but have this be substituted for a magic constant at runtime.

The aim is to make it easy for frontend-aware clients to understand that the user's intent is to refer to the SELF PodId, which might not be clear from the use of a Raw() value.

Roughly speaking, given this POD Request:

REQUEST(
  Equal(?pod_id, Raw(0x5e1f))
  Equal(?pod_id["key"], 123)
)

If the response contains this:

Equal(0x123456, 0x123456)
Equal(0x123456["key"], 123)

Then the client is entitled to reject the response as the literal values do not match.

However, given:

REQUEST(
  Equal(?pod_id, SELF)
  Equal(?pod_id["key"], 123)
)

Then the client should accept the response as SELF is a special value which should match against any PodId.

This issue depends on #345 to define the magic constant.

[ed255]

Just a small note: following the current implementation in #345 the display representation of the raw value of SELF would be:

0x000000000000000f0000000000000001000000000000000e0000000000000005

It appears in reverse because a Value is built with least significant fields first. I think this representation is fine.