Add a warning when "0 of them" is used. Point to some new documentation that
explains why "none of them" is preferred.

Handle some edge cases, which are all now errors:

* -1 of them
* "foo" of them
* /foo/ of them

And this one is also an error if the external variable "x" is a negative integer
or a string:

* x of them

I've also made sure this works with a construct like "0 + x of them" where x is
defined as a negative integer.

Add test cases for as many of these as I can. I don't have test cases for the
external variable case, but it does work as expected. Here is the output of a
rule using external variables where the variable is defined to be 1, 0, -1 and
foo:

```
wxs@mbp yara % cat rules/test.yara
rule a {
  strings:
    $a = "FreeBSD"
  condition:
    x of them
}
wxs@mbp yara % ./yara -d x=1 rules/test.yara /bin/ls
a /bin/ls
wxs@mbp yara % ./yara -d x=0 rules/test.yara /bin/ls
warning: rule "a" in rules/test.yara(5): Consider using "none" keyword, it is less ambiguous. Please see https://yara.readthedocs.io/en/stable/writingrules.html#sets-of-strings-1 for an explanation.
wxs@mbp yara % ./yara -d x=-1 rules/test.yara /bin/ls
error: rule "a" in rules/test.yara(5): invalid value in condition: "-1"
wxs@mbp yara % ./yara -d x=foo rules/test.yara /bin/ls
error: rule "a" in rules/test.yara(5): invalid value in condition: "string in for_expression is invalid"
wxs@mbp yara %
```

If you specify "2 of ($a)" it is now a warning because it will always evaluate
to false, and is quite possibly not what you wanted to write. This is true for
string sets, rule sets and string sets in a range like "2 of ($a) in (0..10)".

If you are using an integer range it is now an error if the lower bound is
greater than the upper bound.

To support rule sets I needed to modify yr_parser_emit_pushes_for_rules() to
use a count parameter, which it will update with the number of pushed rules.
This is identical to how yr_parser_emit_pushes_for_strings() works.

While I'm here, remove the link from the warning message for the "none" keyword.