This will provide methods to manipulate a rule. It is separated from a
grammar in order to simplify it. Essentially it is just a wrapper for
two arrays.

This would shadow previous errors.

There will be no setter.

A wrong check for realloc failure will cause a segfault, on addition of
new rules, because the old pointer address is still left intact, even if
it was freed.

Changing whether a symbol is a terminal symbol affects the consistency
of the grammar, as nonterminals are required in some places. Also
changing a symbol's type effectively changes it's identity, so it is
sensible to not allow it at all.

Removing a symbol, that is used, is problematic.