Revisit type conversions, fix documentation and filling bug

I added log with changes to `RELEASE_NOTES`:

 * Operations GetAs, TryAs (ObjectSeries), GetColumns, GetRows, GetAllValues, ColumnApply (Frame)
  and filling of missing values uses "safe" conversion (allows conversion to bigger numeric type)
 * Avoid boxing when filling missing values (#222)
 * Fix documentation bugs (#221, #226) and update formatters from FsLab

The three commit messages have all the details. The change with "safe" conversion is the most notable one, so here is the commit message:

> This change was triggered by my surprise that `fillMissingWith 0.0` did not work on frame I loaded from a CSV file (because the values were loaded as `decimal` and 0.0 is `float` - and so they are of different types).

> The checkin implements nicer conversion function that has three modes (exact, safe and flexible), replacing castType (exact) and changeType (flexible). Safe can be used to allow safe widening in numerical conversions (e.g. converting int/float32/decimal to float or byte to int/decimal/float). The checkin uses the new safe mode in a few places, but it lets us be more careful about how hidden conversions are done in the future.