Add a Clojure FFI so libraries can bind native code (jolt.ffi)

A jolt library can now bind its own native dependencies and expose a Clojure API
over them — no jolt built-in required. This is the foundation for moving the
http-client / db / adapter functionality out of the host and into real libraries.

- jolt.ffi/foreign-fn (sugar: defcfn) is a compiler special form: a compile-time
  -typed C signature lowers to a real Chez foreign-procedure (analyzer :ffi-fn ->
  backend foreign-procedure), so calls are typed and marshaled, not eval'd.
- host/chez/ffi.ss provides the rest under jolt.ffi: load-library, alloc/free,
  read/write/sizeof, ptr<->string, null/null?. Loaded after the loader snapshot
  so a library's (require '[jolt.ffi]) still loads the macro side.
- Types: int/uint/long/ulong/int64/uint64/size_t/ssize_t/iptr/uptr/double/float/
  pointer/string/void/uint8/char.

Validated end to end: a pure-Clojure file binds libc (getpid/strlen/abs) and
libsqlite3 (open/prepare/step/column/finalize over out-param pointers) and runs a
query. Gate test test/chez/ffi-binding-test.ss (make ffi); selfhost holds.

Makefile

@@ -45,6 +45,7 @@ sci:
 # FFI + threading: HTTP server GC-safety (blocking calls deactivate the thread)
 # and http-client temp-file uniqueness, plus a live request.
 ffi:
+	@chez --script test/chez/ffi-binding-test.ss
 	@chez --script test/chez/ffi-server-test.ss
 
 # Transients: mutable backing, snapshot on persistent!, and linear-time builds.