ondisk: Default payload_size if not given.

If I try to make it const fn, I get

error[E0015]: cannot call non-const fn `<usize as std::convert::TryInto<u8>>::try_into` in constant functions
    --> /home/dannym/src/Oxide/config/amd-apcb/src/ondisk.rs:4564:65
     |
4564 | ...                   (size_of::<$struct_>() - 2).try_into().unwrap()
     |                                                   ^^^^^^^^^^
     |
     = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
     = note: this error originates in the macro `impl_EntryCompatible` which comes from the expansion of the macro `crate::struct_variants_enum::collect_EntryCompatible_impl_into_enum` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0015]: cannot call non-const fn `std::result::Result::<u8, TryFromIntError>::unwrap` in constant functions
    --> /home/dannym/src/Oxide/config/amd-apcb/src/ondisk.rs:4564:76
     |
4564 | ...                   (size_of::<$struct_>() - 2).try_into().unwrap()
     |                                                              ^^^^^^^^
     |
     = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
     = note: this error originates in the macro `impl_EntryCompatible` which comes from the expansion of the macro `crate::struct_variants_enum::collect_EntryCompatible_impl_into_enum` (in Nightly builds, run with -Z macro-backtrace for more info)

That's too bad.

oxidecomputer / amd-apcb

ondisk: Default payload_size if not given. #83