pkl: unit types

[amitu]

So in pkl you can write timeout = 5.ns (Duration type) or disc-space = 5.gb (Data Size type). It's quite funny that these are the only two "unit types" they have. Other types are the usual suspects, String, Integer etc. They do not even have a DateTime type.

We already support timeout.ns: 5 and disc-space.gb: 5 syntaxes. But they are not great for arithmetic, you can not say 5.km + 3.m for example.

We should support a special casing of enums, that resolve into decimals, to allow timeout: 4.s + 200.ms etc. Not sure.