Extend power of the each form

This changes the implementation of the `next` function which
is now used to implement each. This let's us iterate over
more types, not just tables and structs.

src/core/asm.c

@@ -87,6 +87,8 @@ static const JanetInstructionDef janet_ops[] = {
     {"in", JOP_IN},
     {"jmp", JOP_JUMP},
     {"jmpif", JOP_JUMP_IF},
+    {"jmpni", JOP_JUMP_IF_NIL},
+    {"jmpnn", JOP_JUMP_IF_NOT_NIL},
     {"jmpno", JOP_JUMP_IF_NOT},
     {"ldc", JOP_LOAD_CONSTANT},
     {"ldf", JOP_LOAD_FALSE},
@@ -110,6 +112,7 @@ static const JanetInstructionDef janet_ops[] = {
     {"movn", JOP_MOVE_NEAR},
     {"mul", JOP_MULTIPLY},
     {"mulim", JOP_MULTIPLY_IMMEDIATE},
+    {"next", JOP_NEXT},
     {"noop", JOP_NOOP},
     {"prop", JOP_PROPAGATE},
     {"push", JOP_PUSH},