Asserts for FLOAT and some other types

Sometimes we can't check for a precise value.

* assert_almost_equal(val1 DOUBLE, val2 DOUBLE, precision INT)
* assert_length(val TEXT, len TINYINT UNSIGNED)
* assert_between_text(val TEXT, minval TEXT, maxval TEXT)
* assert_between_int()
* assert_between_double()
* assert_between_length(val TEXT, minlen TINYINT UNSIGNED, maxlen TINYINT UNSIGNED)

All these asserts have a NOT version.
Between assert can have NULL for minval or maxval, if there is no min or max.

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These are needed too, for timestamp/date values:

* assert_[not_]between_datetime(dt1, dt2, msg)
* assert_datetime_[not_]within_margin(dt1, dt2, margin, msg) -- margin example: '1 MINUTE'

These should cover TIME (add to CURRENT_DATE()), DATE (00:00:00 should be assumed), DATETIME (native), TIMESTAMP (converted to DATETIME). NO_ZERO_DATE and NO_ZERO_DATE_PART are ok.