FR: allow schema evolution (in or after 0.6)

I know it would be serving really different audience if it supports schema evolution (SE for short). But I would really benefit from similar encoding as bitcode but with SE and it would be a lot of duplicate code to implement such thing. So I'm here to ask: will you consider adding SE as a flavor of bitcode? Or somehow allow me to write a crate that easily reuses bitcode encoding and add SE?

I think the main thing that schema evolution need is

1. appending fields to struct (the encoding will store the number of fields in struct and new fields will be deserialized to default)
2. appending variant to enum
3. increase capacity of ints (I.e. u8 to u16) (though this is less important)

In 0.6 style encoding, this would not add much space either.

No files, tests, or entry points are named. Start by reading the existing bitcode encoding, especially the 0.6 style, then clarify compatibility and defaults for appended struct fields, enum variants, and increased integer capacity; the work is done only after a concrete schema-evolution design and scope are agreed.