Meta issue for discussion of a lazy API

[leafpetersen]

Do we need to provide lazy casting/parsing? That is, instead of eagerly casting, instead return delegating (and probably memoizing) implementations of the returned type that only cast on reads.

It's not clear that laziness makes sense for the parsing case, but it's possible. For the cast conversion case, converting lazily would be beneficial when reads are sparse, but harmful when reads are dense.

[nex3]

Pub definitely needs this. It's crucial that we not eagerly fail for pubspecs, because they might be written for newer versions of pub than the version reading the pubspec. We generally want to fail only if we actually need to read a part of the pubspec that's invalid.