Indicates that an expression can be selected from a source.
Columns will implement this for their table. Certain special types, like
Bound will implement this for all sources. Most compound
expressions will implement this if each of their parts implement it.
Notably, columns will not implement this trait for the right side of a left
join. To select a column or expression using a column from the right side of
a left join, you must call
.nullable() on it.