Directory: lib/skc_corelib, lib/skc_rustlib, builtin
skc_corelib defines the core classes like Object, Bool, Int together with its methods.
skc_rustlib also defines core methods but written Rust.
builtin/*.sk are Shiika code to define core library.
builtin/*.sk and skc_corelib are compiled into builtin/builtin.bc by shiika build_corelib.
skc_rustlib is compiled by running cargo build (as usual).
shiika run executes clang to link these with user program.