sql: fix antisymmetric boolean comparison in VDBE

There are a few situations when booleans can be compared with values of
other types. To process them, we assume that booleans are always less
than numbers, which in turn are less than strings. On the other hand,
function which implements internal comparison of values -
sqlMemCompare() always returns 'less' result if one of values is boolean
and another one is not, ignoring the order of values. For instance:

... max (false, 'abc') -> 'abc'
... max ('abc', false) -> false

This patch fixes this misbehaviour making boolean values always less
than values of other types.