Categoricals: correctly handle missing values, speed up simple string comparisons

[jpn--]

This PR adds two new features:

• Fix bug in compiling XXX.isna() when XXX is a categorical variable. Previously, the missing value code of -1 was mapped to the last categorical value rather than correctly indicating a missing value. The isna code has been patched to get the correct result in these cases.
• Simple string comparisons against categorical variable are now converted into integer comparisons. So, if XXX is a categorical variable with categories ['Bus', 'Car', 'Walk'], then compiling XXX == 'Car' will be converted essentially to XXX.cat.codes == 1 as 'Car' is category index 1. Compiling XXX == 'Bike', on the other hand, will remain unchanged, and now emit a warning as this expression can never be true.