Don't allow escapes through to output

[dankamongmen]

I'm kinda shocked I never thought about this, but we oughtn't be allowing escape through to output, much like we don't allow NULs or newlines. Pretty obvious reasons.

[dankamongmen]

ESC is a control character, and iscntrl(3) returns true for it.

static int
term_putc(FILE* out, const egcpool* e, const cell* c){
  if(cell_simple_p(c)){
    if(c->gcluster == 0 || iscntrl(c->gcluster)){

so this is already handled.