Fix parsing of logical expressions

This fixes the issue leftover after #73. Currently the precedence of `and` is higher than that of `or`, while they should be equal according to the spec.

Testcase:

```bash
$ cd tools
$ ../deps/luajit/usr/local/bin/luajit pflang-compile '0=0 or 0=0 and 0=1'
```

Result before fix:

```
return function(P,length)
   do return true end
end
```

Result after fix:

```
return function(P,length)
   do return false end
end
```

Also added two more tests.