internal/format: bound header size during Parse

[sweis]

format.Parse accumulates stanza bodies with no cap on total header bytes, so
a crafted header can exhaust memory in an automated age.Decrypt caller before
any MAC is checked.

Fix: wrap the input in a counting reader that errors at 16 MiB. The payload
reader is reconstituted from bufio's overread prepended to the original input
as before; the rr == input short-circuit is removed since the wrapper makes
it unreachable.

Test: go test ./internal/format/ -run TestParseHeaderSizeLimit -v
(feeds an infinite-stanza-body reader; pre-fix this would read forever.)

[FiloSottile]

Opened #701 to discuss this. Feel free to close the PR for now.