Refactor the Reader API

[pvdrz]

The crate::msgs::codec::Reader type is used in src/low_level.rs as it is used to decode messages from a buffer. This Reader is basically a cursor over a buffer, meaning that it stores the index of the most recently read byte.

The managed buffers API uses Reader as it is required to decode messages but it is not possible to access the inner index to update the discard field inside crate::low_level::Status. An offset field was added inside crate::low_level::LlConnectionCommon to keep track of this index by calling Reader::used repeatedly.

It should be possible to deduplicate this by refactoring the Reader type.