lib/core: add no-overhead read_bytes implementation

lib/core: add no-overhead read_bytes implementation

read_bytes as a simple interface is relevant, but may hinder
performances in case a chunk is repeatedly read and discarded as for
each read operation, a buffer is re-allocated.

A secondary read operation is introduced to read directly to a CString,
using closer-to-C semantics.

Also start using codec lookahead as source for reading bytes.

Signed-off-by: Lucas Bajolet <lucas.bajolet@gmail.com>