enhancement(remap): improved type definitions for or with nullable fields

[StephenWakely]

Replacing #6344 - I've updated it to work with the recent parser changes.

Following from #6182, a number of functions return fields that can be Null, for example appname from parse_syslog can be Kind::Bytes | Kind::Null.

This PR makes it possible to specify a default value using Or. Since we know that if the lhs is either Null or Bytes, we can deduce that the whole expression lhs | rhs will either be the lhs - Kind::Null or the rhs.

Eg. This now typechecks:

.parsed = parse_syslog(.message)
.sha = sha3(.parsed.appname || "default")

However this will still not typecheck, the procid field can be either a string, an integer or null. So even after an Or it could still be an Integer or a String.:

.parsed = parse_syslog(.message)
.sha = sha3(.parsed.procid || "default")

Signed-off-by: Stephen Wakely [email protected]

[JeanMertz]

Sweet! Left a few comments, but otherwise this looks good to go.