Improve flychecking.

Flychecking will now work correctly with arity checking, and
will better handle imports. Well structured modules should interact
cleanly with the flychecker in a mostly safe manner, but maliciously
crafted modules can execute arbitrary code. As such, the flychecker is
not a good way to validate completely untrusted modules.

We also extend run-context with an :evaluator option to replace
:compile-only. This is more flexible and allows users to create their
own flychecker like functionality.

src/core/compile.c

@@ -454,7 +454,6 @@ static JanetSlot janetc_call(JanetFopts opts, JanetSlot *slots, JanetSlot fun) {
                 break;
                 case JANET_CFUNCTION:
                 case JANET_ABSTRACT:
-                case JANET_NIL:
                     break;
                 case JANET_KEYWORD:
                     if (min_arity == 0) {