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}
(cherry picked from https://android-review.googlesource.com/q/commit:ada3d9a04b3512674f059ca501ef2a1bd5353dc6)
Merged-In: Ifa6f822df6aba06bcf13b8694ea182d7f1cfac1d
Change-Id: Ifa6f822df6aba06bcf13b8694ea182d7f1cfac1d