Explicit struct layout

[daurnimator]

oops; I accidentally created this issue before I wrote it out. Bear with me for a couple of days as I finish the proposal

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packed struct doesn't give enough control
Explicit layout by declaring bitOffsetOf/byteOffsetOf
Cap n proto
Overlapping fields

Alternative to #5049
This proposal may be adding too much complexity, #383 might be a better solution.

[andrewrk]

Feel free to edit & reopen when it's done