lisp: Rust eval() built-in should not invoke the expander

[jorendorff]

Very weird symptom: if you trigger a syntax error in the repl, then exit the repl, you get an extra error message.

» ()
error: car: pair required
» ^D
Error: #(error syntax error #<unspecified> ())

The cause is that the eval() builtin, which is implemented in Rust, calls the sc-expand procedure, which is written in Scheme, and sc-expand raises an error. The REPL's error handler invokes an escape continuation.

Now, if (eval) were written in Scheme, invoking that continuation would cause the (eval) stack frame to be discarded. But it's not. So we get this really weird behavior: the Scheme stack is replaced by the continuation, but the Rust stack is unaffected.

The root cause is that the stack has this pattern: Scheme -> Rust -> Scheme. In this case, the innermost Scheme code can call a continuation and get very odd behavior. We have to prohibit recursing into the interpreter.