pdl: Use ‘bytes’ crate to read scalars

Before we parsed the scalars by hand by tracking an offset and picking
out individual byte indices. Now we delegate this to the bytes crate.
This simplifies the generated code significantly (the bytes crate was
already a dependency of the generated code).

Test: atest pdl_tests pdl_rust_generator_tests_{le,be}
Change-Id: Ifa6f822df6aba06bcf13b8694ea182d7f1cfac1d