Implement LibAFL-based fuzzer

[gtrepta]

Introduces skribe-fuzz, a LibAFL-based fuzzer binary with stubs for coverage.

Changes:

• Add libafl and libafl_bolts version 0.15.4 as workspace dependencies
• Add skribe-fuzz binary with --fuzz-spec, --contract-name, --function-name, --workspace, and --iterations flags
• Add stubs for get_coverage_size and write_coverage_data
• As a workaround for increased K configuration term depth due to coverage tracking, spawn fuzzer on a thread with 64 MB stack size
• Extract FuzzConfig, SignatureFuzzer, and extract_template_and_signature into dedicated modules
• Bump kframework-rs from 73c4986 to a3ca113
• Add rust-toolchain.toml, pinning to Rust 1.95.0