Remove bincode in Favor of postcard for Serialization

[0xmuon]

Description

This PR removes the use of the bincode crate from the concolic observer module and standardizes all serialization in LibAFL on the postcard crate.
Replaced all bincode usage in libafl/src/observers/concolic with postcard.
Removed the bincode dependency from libafl/Cargo.toml.
This change improves consistency across the codebase and leverages postcard's no-std readiness and smaller output sizes.
Closes: #3273

NOTE: Besides converting from bincode to postcard,changes to libafl/src/observers/concolic/serialization_format.rs include:

• Removed all bincode-related configuration, setup, and function calls (such as serialization_options, deserializer_config, and encode_into_std_write).
• Removed the use of custom serialization configuration objects, making the code more straightforward and less error-prone.
• Switched from streaming/deserializer-based logic to buffer-based logic for reading and writing messages, using postcard’s from_bytes and to_allocvec.
• Simplified error handling for serialization/deserialization, now matching on postcard’s error types.
  Removed Unused Code:
• Eliminated now-unnecessary fields and functions related to bincode (such as serialization_options, deserializer_config, and related struct fields).

Checklist

• I have run ./scripts/precommit.sh and addressed all comments

[tokatoka]

can you make the code build?

[tokatoka]

you SURE that this is what you want to do?