Introducing a compiler options system, so we can control diagnostics and such. We begin with three options for diagnosing shadowing. * autoload.c (compiler_set_entries): Register a structure name compiler-opts, a with-compile-opts function name, *compile-opts* variable name, and slots shadow-fun, shadow-var and shadow-cross. * stdlib/compiler.tl (compile-opts): New struct. (%warning-syms%): New macro. (*compile-opts*): New special variable. (when-opt, with-compile-opts): New macros. (opt-controlled-diag): New function. (env extend-var): Call extend-var* method instead of repeating code. (env extend-var*): Implement shadow-var and shadow-cross diagnostic options. (env extend-fun): Implement shadow-fun and shadow-cross diagnostic options.