parse variable names that begin with ordered

[randommm]

I don't know if this issue already exists, but any variable name that begins with ordered can be declared, but cannot the assigned or used in sampling statement.

For instance, the following parses:

stanc(model_code="
parameters {
  real orderedanything;
}
model {

}
")

But this doesn't:

stanc(model_code="
parameters {
  real orderedanything;
}
model {
  orderedanything ~ normal(0, 1);
}
")

[bob-carpenter]

That's a bug. I assigned it to myself with high priority.

• add warning for "ordered" as prefix on variables
• add doc that ordered is unallowed as a prefix