Add an IR for BTOR2 to reduce the dependencies on the C parser

tools/btor2/btor2i/.gitignore

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+# Generated by Cargo
+# will have compiled files and executables
+debug/
+target/
+build/
+
+# Remove Cargo.lock from gitignore if creating an executable, leave it for libraries
+# More information here https://doc.rust-lang.org/cargo/guide/cargo-toml-vs-cargo-lock.html
+Cargo.lock
+
+# These are backup files generated by rustfmt
+**/*.rs.bk
+
+# MSVC Windows builds of rustc generate these, which store debugging information
+*.pdb
+
+# MacOS DS_Store files
+**/.DS_Store
+
+# VSCode files
+.vscode/
+
+# IntelliJ files
+.idea/
+
+# Build directory
+build/