pdl: Add support for scalar and enum arrays

This implements support for integer and enum arrays with static and
dynamic sizes.

The parse function on the data structs changed its argument from
‘&[u8]’ to ‘Cell<&[u8]>’. This allows us to pass around a single slice
which we mutate in-place. The ‘Cell’ type is a no-overhead type which
cannot panic, so this simply allows us to mutate the slice in-place.

The in-place mutations allow us to eat away the slice as we parse. The
alternative would be to make all parse functions return two values:
the parsed result and the remaining slice. That would in turn
complicate the logic where we collect results.

Another alternative would be to make the parsing stateful by storing
the slice in a struct (and that’s actually what ‘Cell<&[u8]>’ does).

Test: atest pdl_tests pdl_rust_generator_tests_{le,be}
Change-Id: Idc13ab75c5d6c4704f7edb70306be761ee815cab