Update verifier

[Alan-Jowett]

This pull request includes several changes to the libfuzzer/libfuzz_harness.cc file to enhance the verifier's functionality and improve code clarity. Additionally, it updates the external/ebpf-verifier submodule.

Enhancements to verifier functionality:

• Introduced stored_invariants as a global std::optional<Invariants> to store pre-invariant data. (libfuzzer/libfuzz_harness.cc)
• Added logic to convert instruction sequences to control-flow graphs and verify programs using stored invariants. (libfuzzer/libfuzz_harness.cc)
• Enhanced the ubpf_debug_function to use the new invariant checking mechanism and provide detailed error messages. (libfuzzer/libfuzz_harness.cc)

Code clarity improvements:

• Refactored options to use verbosity_opts for better organization. (libfuzzer/libfuzz_harness.cc)
• Replaced the label string with a crab::label_t type that includes a stack frame prefix for better readability. (libfuzzer/libfuzz_harness.cc)

Submodule update:

• Updated the external/ebpf-verifier submodule to the latest commit. (external/ebpf-verifier)

[coveralls]

coverage: 78.848%. remained the same
when pulling 796f1be on Alan-Jowett:update_verifier
into 16d52a4 on iovisor:main.